Do mathematical objects disappear? I am asking this question starting from two orders of considerations. 
Firstly, we can witness, considering the historical development of several sciences, that certain physical entities "disappeared": it is the case of luminiferous aether with the surge of Einstein's relativity, or the case of the celestial spheres that disappeared in the passage from the pre-copernica universe to the Copernican universe.
Secondly, if we consider the evolution of mathematics, we assist to quite an opposite phenomenon: new mathematical objects are often invented and enrich already existing ontologies (let us consider the growth of the number system with negative, imaginary, etc., or the surge of non-euclidean geometries).
But can anyone think of examples of mathematical objects that have, on the contrary, disappeared (have been abandoned or dismissed by mathematicians)? 
 A: Mathematical objects sometimes disappear when proven either non-existing or isomorphic to more well-known objects (such as "Maschler spaces").
A: One of my favorites: the Oxford English Dictionary defines aliquot as "Contained in a larger number a certain number of times without leaving any remainder; forming an exact divisor", and the term is in use in elementary number theory.  But the antonym, according to the OED, is aliquant: "Contained in a larger number but not dividing it exactly", for example,

1695   W. Alingham Geom. Epitomiz'd   "An Aliquant part is a lesser
  Number in respect of a greater, when it doth not measure it exactly,
  as 3 is an aliquant part of 7."

A: There are cases like this ...
Walter Feit and John Thompson wrote a 250-page paper on nonabelian simple groups of odd order, and in the end conclude that there are none.
A: Perhaps mathematical objects can be said to "disappear" when they were first thought to be consistent, but were later proven to be inconsistent. An example of this is excessively
hypercompact cardinals.
A: Newton's fluxions were abandonned after Leibnitz notion of infinitesimally small numbers proved to be more manageable.
A: Quaternions haven't disappeared, but they are much less commonly used than they used to be. At one time they were commonly used for the tasks that we would today accomplish using vector calculus. Historian Michael J. Crowe depicts quaternions and vector calculus as being in a kind of Darwinian struggle, which quaternions lost.
A: Mathematical objects do not disappear, except possibly in the rare cases when something was named and then proved that it does not exist. Mathematical objects come into and out of fashion from time to time, but they almost always come back.
Terminology changes of course, but the things remain, whether they have special names or not. 
Let me give some simple examples from trigonometry. Of course trigonometry is not a fashionable research subject anymore, so some terminology is forgotten.
David Handelman gives "haversine" as an example. The term is forgotten by mathematicians. But there exists a useful "haversine formula" in astronomy,
and unlike aether in physics, the thing itself did not disappear.
Ancient Greeks did not have our trigonometric functions using $\mathrm{chd}\, x$,
the chord corresponding to angle $x$ instead. As an independent trigonometric function it disappeared. But the notion of the chord did not disappear. Sometimes this function is indeed handy in trigonometric calculation. When necessary it is
resurrected:-)
http://arxiv.org/abs/math/0009251
A: This does indeed happen. One very clear example is that of a naive set, i.e. a set conceived of as a collection of every object that has some particular property, without any further restrictions on its definition. This was in common currency before Russell's paradox came to light. (Russell's paradox is this: consider the set of all sets that do not contain themselves. Is this set a member of itself or not?) Afterwards it was replaced by the much more refined notion of sets as per ZF set theory (or ZFC), in which the notion of "the set of all sets that do not contain themselves" cannot be expressed.
This example suggests a major difference between mathematics and physics. In physics, a theory can simply be wrong, in the sense of not corresponding at all to reality. For example, the luminiferous ether was not the correct explanation for light waves, and when special relativity came along it had to be discarded entirely. However, in mathematics the only way to be wrong is to be inconsistent. (If a mathematical theory fails to correspond to reality it might lose some applications, but if it's interesting in its own right people will continue to work on it.) As in the case of naive set theory, an inconsistent theory can often be 'patched up' and made consistent in such a way that the core results are preserved. So in physics, the 'disappearance' of a theory can be a rather catastrophic event that affects the whole field, whereas in mathematics it's more likely to take the form of a small change, and be of interest only to a small group of researchers.
A: I would nominate the first apotome of a medial.
A: I certainly can't think of examples similar to your physics examples of concepts that were just wrong so effectively became extinct.
Two extremes which are present in mathematics are
1) Things which become too simple to have their name retain prominence as commonly known terminology. 
For example: For Aristotle, Square numbers and Oblong numbers were perhaps similarly important. Today the first concept is vibrant while second concept is fairly unfamiliar. Similarly the concept of singly and doubly even integers is fairly unfamiliar being subsumed as the case $p=2$ of the $p$-adic order of an integer (or rational number.)
AND
2) Things which are too complex for the tools of their time and so hibernate for a while
As an example, I feel compelled to quote the stirring first paragraph of The Invariant Theory of Binary Forms 
Like the Arabian phoenix rising out of its ashes, the theory of invariants, pronounced dead at the turn of the century, is once again at the forefront of mathematics. During its long eclipse, the language of modern algebra was developed, a sharp tool now at long last being applied to the very purpose for which it was intended. More recently, the artillery of combinatorics began to be aimed at the problems of invariant theory bequeathed to us by the nineteenth century, abandoned at the time because of insufficient combinatorial thrust.
A: I never see references to the haversine (used in spherical trigonometry, itself an obsolescent, if not obsolete, subject). Also, versine and coversine (and their variants) are obscure (and unnecessary) trigonometric functions, and have been for at least fifty years.
A: I believe that the closest analogues to your physical examples arise when mathematical texts are, at the time they are written, regarded as being precise, but are later regarded as being insufficiently precise or rigorous, so that the nouns in the older text are seen as requiring reformulation in "modern, rigorous terms" before they are accepted as directly and unambiguously referring to some mathematical entity.
For example, near the beginning of Euclid's Elements we read, "A surface is that which has length and breadth only."  Today, many mathematicians would instinctively react, "Say what?"  Before they can make sense of Euclid's definition of a surface, they need to reformulate it in modern language.  Now, once they do so, they will deny that Euclid's surfaces have "disappeared," because according to the standard platonistic philosophy that forms the basis of the working mathematician's conception of the world, the universe of mathematical objects is an eternally existing thing, and it is impermissible to declare that objects in that universe cease to exist.  Rather, we say instead that Euclid used language in an old-fashioned manner, and that we need to translate his text into modern language before we can reliably determine the referents of his nouns.  Once the referent of the noun is established, then it lives forever and cannot disappear; the only things that can "disappear" are ways of speaking that are later judged to be not clear enough to unambiguously describe eternally existing realities in the mathematical universe.
If, however, one takes a less platonistic view of mathematics, then one might simply say that Euclid's surfaces (and other concepts) in their original form have disappeared, but that the essential mathematical content of the Euclidean theory has been preserved.  In fact, if the modern reformulation is done carefully, much of the older text can be preserved verbatim.  There will, however, be some instances where the older terminology and sentences lack a one-to-one correspondence with the modern version; older concepts that are judged to be superfluous or vague are just dropped.  In this sense some things "disappear," and it's not just that certain words have fallen into disuse, but the concepts that they were supposed to refer to are deemed insufficiently precise or rigorous.
As others have mentioned, infinitesimals are another example of this kind of "disappearance."  In mathematics proper (as opposed to science and engineering), infinitesimals as they were conceived in classical calculus no longer exist in mathematics, in the sense that the concept that people used to have is no longer considered to be rigorous or precise enough.  Obviously, the essential mathematical content has been preserved in modern treatments (whether "standard" or "nonstandard" analysis), and some theorems can be preserved verbatim when the words are suitably redefined.  But there has been a definite shift in the underlying concepts, even though "no matter has been created or destroyed" in the platonistic universe of all mathematical objects.
A similar story can be told in other cases where there has been a change in "rigor."  Algebraic geometry may be another example where so-called classical Italian algebraic geometry was later regarded by some as lacking in rigor.
A: It's certainly possible for mathematical terminology to disappear. For instance, we used to have specialized words for different powers of numbers. We still use square and cube in this way, but for higher powers we would now say "[number]th power" rather than say the zenzizenzizenzic for the eighth power.
A: Hopefully at some point, GRH will be proved, and then the Siegel zeros of Zeta functions will disappear. There are currently thousands of papers on them.
A: It's a great question! The question is a bit "vague" - because you really don't specify "forgotten by who"? Let's assume that you mean "forgotten by the main stream math community at a given time" (because - really once a result is documented in print - it never is really forgotten - only the number of people aware to its existence can vary - there is probably some threshold that makes a result "common knowledge" - where "common" refer to communal).   
I cannot think of examples - of "objects" (as in - definitions) that have appeared and disappeared - but I can think of examples of a few results (Theorems) that have been discovered - generally forgotten - and later rediscovered! There are a few examples like that in number theory - some results even quite fundamental. For instance results discovered by Eular - forgotten -
 later rediscovered in 19-th century - forgotten - later rediscovered circa 1940's - later rediscovered circa 1990's - and every time with different understanding and insight on their contents - so it happens. 
Also there is an interesting description of such a process in an Appendix of the very classical book by Gelfand, Kapranov & Zelevinsky (GKZ) on the concept of determinants of complexes required for the computation of discriminants - which was apparently already discovered by Cayley in the early days of modern algerba! The way they write about it is really quite an interesting and recommended read (their modern exposition relies on heavy use of homological algebra). So that might be another example for a historical process of the kind you are asking for.
A: In Mathematics everyone believed that  Hilbert's second problem had an affirmative answer but it was (later) shown that this is not entirely true  and I suppose that the consistency of arithmetic has now been "disappeared"   
A: Before trying to make parallels, you must understand how these other concepts "disappeared". 
The point of science is to create theoretical models which allow us to do some reasoning. The goal of natural sciences, such as physics and biology, is to explain how entities in the world around us function. A physics model is only useful when it either 


*

*does not contradict any observations

*contradicts observations in known ways and is usable for answering questions on a desired level of detail. For example, a gravitational model of the Milky Way can assume that each planet is a sphere, while it is known that planets are flattened spheres. 


Such models are considered "right" in physics, while the ones which contradict observations are considered "wrong". 
Unlike physics, mathematics does not have the primary goal of explaining natural phenomena. While its theoretical models can be applied to entities found in nature, for example arithmetic, they are considered interesting for their own sake even when they don't describe anything we have observed, see for example the Banach Tarski theorem. 
Thus, the mechanism which makes some theories in natural sciences wrong (ether in physics, Lamarckian inheritance in biology) does not apply to mathematics. It is impossible for postulated mathematical entities to disappear the same way, because in mathematics, an entity is already a part of a useful model when it is described, not only when it has been found to correspond to an entity in reality. 
As others have noted, it is possible for mathematical entities to fall out of fashion. But while they are seen as old and unnecessary baroque, they are not seen as a direct misconception, the way it is with the disappeared physics entities. 
A: From A. A. Ivanov (The Monster Group and Majorana Involutions):

[$\dots$] John Conway suggested calling the extensions of $^2E_6(2)$ the Baby Monster, the double extension the Middle Monster, and the triple extension the Super Monster. Fischer was not in favour of this terminology being used externally. When it was shown that $^2E_6(2)$ can only be extended twice and therefore that the Super Monster does not exist, the prefix Middle was dropped and the name Monster, as we know it, emerged. [$\dots$]

Consequently, the Super Monster is an example of a mathematical object that was named but later disappeared in a puff of nonexistence.
A: Screws and Screw Theory.
I own a copy of the book A Treatise on the Theory of Screws by Robert Ball published around 1900. The algebra of screws, and the related twists and wrenches seem to have played an important role in the study of rigid body dynamics at the time. But that terminology seems to play very little role in any writing on that subject that I've seen more recently. Except in around 2000 the terminology picked up again because of applications to robotics and to rigid body simulation (that I learnt about via simulations in the video games and visual effects industry).
A: Yes, definitely.
Sometimes the construction become obsolete.  For example, in Mumphord's little red book we read about pre-scheme, but essentially nowhere in modern mathematics are "pre-"schemes studied.  
Also, there are object which are just too non-mainstream at the time of their conception and never really catch on.  Sadly they become forgotten, even though maybe they are ingeritly very interesting and unique objects.  For example there is the concept of a Cosmos, which is already forgotten by many mathematicians due to it's inaccessibility (and non-mainstream-ness).  
