This is the name given to a 4-chord sequence played as a jam by the Grateful Dead. It is thought by some to be related to the Paul Kantner song "Your Mind Has Left Your Body." The title "Mind Left Body Jam" was - I think - originally used by DeadBase. The first Grateful Dead CD to include a version was "Dozin' At The Knick", where the title was "Mud Love Buddy Jam" in a humorous reference to the DeadBase/taper title. But subsequent releases have adopted the "Mind Left Body Jam" title.
(1) a track from 26 June 1974 is listed as "Mind Left Body Jam" in the liner notes ("Mud Love Buddy Jam" on the back cover of the CD), though a different track from 28 June labelled simply as "Jam" contains a segment closer to what is normally known as "Mind Left Body Jam"
(2) called "Mud Love Buddy Jam" on the CD cover and liner notes