Mathematical objects whose name is a single letter (Not research-level, but perhaps not easily answered elsewhere — you decide if MO can afford the innocent fun. If so, it should likely be “community-wiki” i.e. one object per answer.)

I am seeking stories of mathematical objects that, in context, eat out namespace because their (most usual) name literally is a letter (e.g., in calculus, $e$).

Per discussion in the comments, please rather exclude letters that are frozen out by being merely common notation ($e$ in group theory, $g$ in Riemannian geometry, whole alphabets in semisimple Lie theory), and not really the name of any single object. But include: how such (poor?) practice came about; what did or didn’t help reclaim letters (new names, new typography,...); or any good story.
Wikipedia’s disambiguation pages can suggest many ($c$, $e$, $i$, $j$, $k$, $o$, $q$, $t$, $F$, $G$, $J$, $K$, $L$, $O$, $P$, $W$, $Y$, $\mathcal O$, $\wp$, $\delta$, $\zeta$, $\eta$, $\vartheta$, $\varkappa$, $\lambda$, $\xi$, $\pi$, $\sigma$, $\tau$, $\chi$, $\mathrm B$, $\Gamma$, $\mathrm H$, $\Xi$, $\Omega$,...), but I am sure that is not all.
 A: To get the ball rolling with actual answers...
$\mathfrak{c}$ is the Cardinality of the continuum.
A: How about the plethora of symmetric functions and generalizations?

This is just a small overview of the various generalizations of Schur polynomials, and related bases of symmetric (or quasi-symmetric) functions.
Furthermore, the notation is not completely standardized.
It does not exactly answer OP's question, but some of these at least are standardized (say the $e_\lambda$, $m_\lambda$).
I myself wonder a bit of the practice in this area.
A: Imagine how different would be our mathematical discourse if students struggled
with $\alpha$-$\beta$ proofs rather than $\epsilon$-$\delta$ proofs!
From an earlier MO question: "Why do we use $\epsilon$ and $\delta$?,"
an answer quoting Judith Grabiner:

$\epsilon$ corresponds to the initial letter in the word "erreur" (or "error"), and Cauchy in fact used $\epsilon$ for "error" in some of his work on probability."

As @RyanBudney summarizes,
"So it's  $\epsilon$ for error in the answer, and presumably $\delta$ is in reference to difference in the input variables."
Or, as @SimonRose says, "Or possibly that $\delta$ is just the next letter over..."
A: In set theory, $V$ is universally known as the universe of all sets.
Similarly, meanwhile, $L$ is the constructible universe.  
A: Trivial example, but not to be omitted, the Greek and Latin numerals. E.g.:

A: A lovely story of $\nabla$ can be found on Wikipedia

Introduced by Hamilton in 1837, his initial notation was ◁. Quotes from Wikipedia: 

The name comes, by reason of the symbol's shape, from the Hellenistic Greek word νάβλα for a Phoenician harp, and was suggested by the encyclopedist William Robertson Smith to Peter Guthrie Tait in correspondence.

(...)

After receiving Smith's suggestion, Tait and James Clerk Maxwell referred to the operator as nabla in their extensive private correspondence; most of these references are of a humorous character. C. G. Knott's Life and Scientific Work of Peter Guthrie Tait:

It was probably this reluctance on the part of Maxwell to use the term Nabla in serious writings which prevented Tait from introducing the word earlier than he did. The one published use of the word by Maxwell is in the title to his humorous Tyndallic Ode, which is dedicated to the "Chief Musician upon Nabla," that is, Tait.


A: Amir Alexander's 2014 book Infinitesimal says that John Wallis introduced the symbol $\infty$ for infinity (p.280). I asked on HSM,
"Where did John Wallis get the idea for $\infty$?,"
and user Conifold said that the source might be
"the Egyptian ouroboros symbol, snake biting its tail":

          


          

(Image from mythologian.net.)


A: Given that there is not much difference between a letter and a symbol (especially in view of the two most upvoted answers), I propose:
• The letter "+" is used for the mathematical object commonly called addition.
• The letter "-" is used for the mathematical object commonly called subtraction.
• The letter "×" is used for the mathematical object commonly called multiplication.
Etc.
