The process of returning to a previous version of installed software -- e.g. from a custom ROM to the original stock ROM for warranty issues, or from a newer Android version to an older one.
While the most common cases involve the Android system (the most important one probably being a reversion to Stock-ROM to prepare the device for warranty service), a "downgrade" is not necessarily resticted to this. One could also downgrade apps to previous versions, just to give an example -- which might be necessary if the installed version causes problems, and no update is available.
