To lock an evolving software distribution or document against changes so it can be released with some hope of stability. Carries the strong implication that the item in question will `unfreeze' at some future date. "OK, fix that bug and we'll freeze for release."
There are more specific constructions on this term. A `feature freeze', for example, locks out modifications intended to introduce new features but still allows bugfixes and completion of existing features; a `code freeze' connotes no more changes at all. At Sun Microsystems and elsewhere, one may also hear references to `code slush' -- that is, an almost-but-not-quite frozen state.