1. /dif/ n. 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. From Jargon Dictionary

2. File comparison utilities The diff package provides the diff, diff3, sdiff, and cmp programs. `diff' shows differences between two files, or each corresponding file in two directories. `cmp' shows the offsets and line numbers where two files differ. `cmp' can also show all the characters that differ between the two files, side by side. `diff3' shows differences among three files. `sdiff' merges two files interactively. The set of differences produced by `diff' can be used to distribute updates to text files (such as program source code) to other people. This method is especially useful when the differences are small compared to the complete files. Given `diff' output, the `patch' program can update, or "patch", a copy of the file. From Debian 3.0r0 APT