Projects often have more than one major line of development. For example: a project might have the current development focus (its trunk), a couple of older releases that are still supported and the current stable release. 

Launchpad calls each of these lines of development a "series". Each series is semi-autonomous, in that they can have their own translations efforts, their own branches of code and bugs can be marked as affecting one or more series within a project.

[[Projects/SeriesMilestonesReleases|More about series]].