Definitions for "Jack stud"
Keywords:  header, stud, rough, nailed, trimmer
A partial stud nailed next to full studs to support the header at door (and some window) openings.
Framing members, generally 2" x 4"s, which form the inside of the window or door rough opening. They run from the sole plate to the header, which is supported by them.
Short stud, which does not reach from the floor to the ceiling, but only from the floor to a window, for example.