Feature Toggle[1] is a technique in software engineering that attempts to provide an alternative to maintaining multiple source code branches.
The technique allows you to release a version of a product that has unfinished features. These unfinished feature are hidden (toggled) so they do not appear in the user interface. This allow many small incremental versions of software to be delivered without the cost of constant branching and merging.
Whilst the pattern can be implemented very simply in most programming languages there are libraries [2]available to further simplify usage.