1. A change listing, especially giving differences between (and additions to) source code or documents (the term is often used in the plural diffs). “Send me your diffs for the Jargon File!” Compare vdiff.
2. Specifically, such a listing produced by the diff(1) command, esp. when used as specification input to the patch(1) utility (which can actually perform the modifications; see patch). This is a common method of distributing patches and source updates in the Unix/C world.
3. v. To compare (whether or not by use of automated tools on machine-readable files); see also vdiff, mod.