Trunk

From ReactOS

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.

Often main developer work is done to trunk and stable versions are branched and ocassional bug-fixes are merged from branches to trunk. Development (and thus production of unstable versions) in non-trunk branches is done for projects that do not change often or change is expected to take long time until it will be ready for incorporating trunk.

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(Taken from Wikipedia's trunk page.)