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:

 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.

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).