A showing meant to prove rather than describe. A demo ends an argument by being watched.
Latin demonstrare (to point out, show fully), from de- (thoroughly) + monstrare (to show), from monstrum (a portent, sign), clipped in modern usage to demo; a showing meant to prove rather than merely describe.
Its root, monstrum, once named an omen strong enough to need no argument behind it. A demo keeps that force: it settles a question by being seen, not by being explained.