A spin-off may be called a sidequel, a portmanteau of "side" (as in side-by-side) and "sequel", when it occurs in the same time-frame as the original, sometimes contacting with the main narrative at points.
A spin-off can be called a sidequel, a blend of "side" (as in side-by-side) and "sequel", when it happens within the same time frame as the original, sometimes interacting with the main narrative at specific points.