1. Checking
code (or anything else, e.g. a
Usenet posting) for completely stupid mistakes. Implies
that the check is to make sure the author was sane when it was
written; e.g. if a piece of scientific software relied on a
particular formula and was giving unexpected results, one
might first look at the nesting of parentheses or the coding
of the formula, as a "sanity check", before looking at the
more complex I/O or data structure manipulation routines, much
less the
algorithm itself.
Compare
reality check.
2. A run-time test, either validating input or ensuring that
the program hasn't screwed up internally (producing an
inconsistent value or state).