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.