Patch file
A ["Patch"] file is a single file annotating the changes between two versions of one or more text files. By "applying" a patch, you can update a set of text files from an older version to a newer version. Developers often use patch files to make their changes available to other developers.
These files are also called "diffs", or a ["Diff"], as they were traditionally generated by the ["Diff"] program. While there are many different formats for patch files, the most common is the ["UnifiedDiff"] format.
Mercurial will generate patch files for you, in the ["UnifiedDiff"] format, with the hg diff command.