TITLE: Is there a reference containing standard mathematical notations?
QUESTION [11 upvotes]: Suppose you are writing a mathematical text (say an article) and you want to call an object (for example, a set) by a letter. It would be cool then to have some reference (optimally available on the internet) where you could find some standard letters and notations of mathematical objects and pick one that you like. Does such a "notation dictionary" exist?
ADDED. Thanks everybody for interesting answers! Maybe it is worth to add that I had in mind rather basic things. The question was trigged by my attempt to find a good letter to denote a subset of the segment $[0,1]$. Finally I decided to call it $T$ (in the course of the proof it turns out that $T$ is equal $[0,1]$ :) ).

REPLY [2 votes]: The International Organization for Standardization (ISO) offers a standard for mathematical notation as part of the more general ISO 80000 standard for quantities and units. It is widely used in the physical sciences and also adopted by the National Institute of Standards and Technology (NIST) in the USA, for example. The full standard can be found around in the internet.
It is a self-consistent very well-thought standard. It generally agrees with common notation in maths literature. It covers areas from basic set-theory, first-order logic, tensors, functional analysis and special functions, and much more. The specialized notation of some areas such as differential geometry or category theory or higher-level formal logic is not quite covered. Notation for probability theory is given separately in the Guide to the Expression of Uncertainty in Measurement.