Trunk

The trunk refers to the unnamed branch - version of a file tree under revision control. Branches are spawned from trunk (or other branch) in order to concentrate on stabilizing the tree for release or to start implementing long-term (possibly throw away) changes that would disturb development in trunk. CVS and other version control systems use the word HEAD instead of trunk. [1]

http://svn.mythtv.org/trac/browser/trunk