Commodore acronym for Movable Object Block, which is the correct technical term for a sprite.
MOBs were what were present in the Commodore 64 and the like. The Amiga also had MOBs, but it also had something called BOBs (Blitter Object Blocks).
I don't remember much about BOBs, but I think the difference was that BOBs were drawn with the blitter and while MOBs were transparent, BOBs needed to include the background.
One Amiga programming book (Amiga 2: Amiga BASIC, by Jyrki J.J. Kasvi) had pictures of criminal "Mobs" running around, chased by "Bobbys"... all of them were, of course, ghosts. (Picture drawn by Wallu, famed Finnish illustrator who's still doing pictures and layout for Finnish computer mags, most notably Pelit).