Patch file
A [:Patch: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: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:unified diff] format.
Mercurial can generate patch files in the unified diff format with the hg diff command.