\begin

The LaTeX command used with \end to delimit an environment within which the text is formatted in a certain way. E.g. \begintable...\endtable.

Used humorously in writing to indicate a context or to remark on the surrounded text. For example:

 \begin{flame}
 Predicate logic is the only good programming
 language.  Anyone who would use anything else
 is an idiot.  Also, all computers should be
 tredecimal instead of binary.
 \end{flame}

Scribe users at CMU and elsewhere used to use @Begin/@End in an identical way (LaTeX was built to resemble Scribe). On Usenet, this construct would more frequently be rendered as "<FLAME ON>" and "<FLAME OFF>" (a la HTML), or "#ifdef FLAME" and "#endif FLAME" (a la C preprocessor).