A film or movie in which the story-line and/or characters were only supporting roles in a previous theatrical release. It is similar to a spin-off
Synonym: a side-story of movie
It differs from a sequel
in which the story-line and characters are assumed to take place chronologically AFTER the original work . It also differs from a prequel
in which the story-line and characters are assumed to take place chronologically BEFORE the original work.
An important characteristic of a sidequel is that the film may not even allude to of any of the principle characters of the original work. It is usually also retains the same director, writer, etc.
The movie, "This Is 40", is a sidequel of "Knocked Up" since it primarily follows the side story of Pete (Paul Rudd) and Debbie (Leslie Mann), the in-laws of Alison Scott (Katherine Heigl) in the original film.