A Sub-series is a series that is entirely included within another series. An example is the Mario Kart series, as every episode in the series is also an episode in the general Mario series.
For a list of sub-series see: Category:Sub-series.
Sometimes sub-series are called spin-offs, but in this wiki we call spin-off only a series that's not included within another series. An example is the Chocobo series, a spin-off of the Final Fantasy series, and it's not included in the Final Fantasy series itself.
So we consider spin-off and not sub-series, a series that has at least one episode not in the original series.
It might also happen that a sub-series becomes a spin-off series after some time. This happens when the sub-series gets one or more releases that can't be included in the parent series.
An example is the Rabbids series of games that was originally created as a sub-series of the Rayman series with the release of Rayman Raving Rabbids and Rayman Raving Rabbids 2, but from Rabbids Go Home afterwards, the series stopped being called "Rayman" and no more featured the Rayman character, so it started being considered a spin-off instead.
If a sub-series gets enough episodes, it might get its own article, divided from the parent series, featuring its own links to other series, also including links to other sub-series of the parent series.