The page you are reading is part of a draft (v2.0) of the "No bullshit guide to math and physics."
The text has since gone through many edits and is now available in print and electronic format. The current edition of the book is v4.0, which is a substantial improvement in terms of content and language (I hired a professional editor) from the draft version.
I'm leaving the old wiki content up for the time being, but I highly engourage you to check out the finished book. You can check out an extended preview here (PDF, 106 pages, 5MB).
http://en.wikipedia.org/wiki/Wikipedia:Mathematical_symbols
Here is a list of commonly used mathematical symbols and what they mean
\equiv
delta
tensor basis in front back
is element of
\[ \delta_{ij} = \left\{ \begin{array}[c] 1 .... \]
not to be confused with \delta – small number used in calculus