It appears to mean a collection of computer code, from an implementation perspective. Users of the word might refer to the "foo-1.0 codebase" as opposed to the "foo-2.0 codebase, where we totally rewrote everything".
Assuming that this is what the word means, it's a useful one. However, where did this word come from, and when? Does it have a more precise meaning, or has it spread across the internet with the rough (but useful) meaning above?
The only definitions I can find are within Wikipedia or Wictionary (neither of which I'm a fan of). I don't see it in the Jargon File either (but ditto, frankly). It also seems to have a (different) specific meaning for Java applets, and is the name of a proprietary database product.
Does anyone have an etymology for this word?