Taken from Wikipedia:
Branching, in revision control and software configuration management, is the duplication of an object under revision control (such as a source code file, or a directory tree) so that modifications can happen in parallel along both branches.
Branches are also known as trees, streams or code lines. The originating branch is sometimes called the parent branch, the upstream branch (or simply upstream, especially if the branches are maintained by different organisations or individuals), or the backing stream. Child branches are branches that have a parent; a branch without a parent is referred to as the trunk or the mainline.