Terminology: Lost in translation or multiple-meanings I was reading Uniformization of Riemann Surfaces by Henri Paul de Saint Gervais (not a real person, but a group of French mathematicians), and the translator kindly points out that the name of "the greatest theorem of the 19th century" comes from the French word uniforme, meaning single-valued (as opposed to multi-valued). It may still seem obscure, but in the text (the introduction) it was explained pretty well.
For a lesser example, in the same introduction it talks about a group $\Gamma$ acting on the upper half plane (fixed point) freely and properly. That casual parenthetical remark cleared up the meaning of "free action" that I could never make sense of and had to look up its definition repeatedly.
I hope it is okay to ask for more examples of this sort. (You are welcome to rephrase the question.) It may not strictly be the original intention, but may have contributed to its wide acceptance but has since been forgotten (e.g. What is the naming reason of poles in complex analysis?). As other Terminology questions and answers show (especially in algebra), this may be quite tortuous to explain properly, and may not be more helpful than "just learn the definition."
For starters: what is so proper about a proper map?
Updated: To avoid being too vague that any terminology can have a story behind it (meromorphic, homology, etc), here are some guiding criteria:
1) Due to translation/importing (most often from French and German), or the multitude of meanings of the English word itself, the original meaning of the terminology has been lost in most texts on the subject;
2) it has been generalized out of the original context;
3) and that the original meaning helps in the understanding of the terminology, without having to give a long explanation.
 A: Poincaré is usually said to be the first one to use homologous. He uses the term in his Analysis situs to mean the relation between manifolds that we nowadays refer to as cobordism or bordism. I have always interpreted his usage to be an extension of the relation of homology (or perspectivity) between triangles in geometry: one can think of a bordism from a manifold M to another N as a (very flexible!) perspectivity from one to the other. 
A: I thought "germ" would be a great example.
English not being my native language, I thought "germs" had to do with bacteria or microbes that cause diseases, which makes no sense in mathematics. I only realized it much later (though I'd like confirmation from others or the literature) that it had to do with the other, perhaps original, meaning: a portion of an organism capable of developing into a new one or part of one. One shall think of "to germinate", given the context of "stalks" and "sheaf/bundle". (There's also the English phrase "the germ of an idea", for what it's worth.)
It apparently was more widely used in function theory (of a complex variable), in which the infinitesimal neighborhood of any point in the domain of a holomorphic function determines the function completely (principle of analytic continuation). One may also find as formal definition of germ at a point as the sequence of Taylor series coefficients at that point. In general, of course, a sheaf in the modern sense may not be flabby/flasque, so that the germ of a section does not determine the section. (Speaking of sections, that's another terminology that may be confusing to some people.)
