Questions tagged relation-symbols should also be tagged symbols: this is a sub-concept.
An exhaustive overview of not only relational symbols can be found in the Comprehensive LaTeX Symbol List.
FAQ
How should I represent mathematical definitions?
Wolfram Mathworld lists three notations for mathematical definitions, namely ≡, :=, and placing a hat on an equals sig; several others are used, such as placing def above an equals sign. Only ≡ is repesented natively as a mathematical relation symbol in core Latex, using \equiv.
The answers to How to typeset $:=$ correctly? give advice on typesetting :=, sometimes used for this purpose, as well as for imperative assignment. The answers to Is there a wider equal sign? treat placing def above =.
