The abbreviation Q.E.D. is short for the Latin quod erat demonstrandum, meaning "which was to be proved".
It is normally used in mathematics once a desired conclusion has been made.
e.g. 3x - 26 = 100 so 3x = 126 therefore x=42 Q.E.D.
a.m. & p.m., cf., e.g., et al., etc., i.e., N.B., P.S., Q.E.D., q.v., viz.
ad hoc, de facto, modus operandi, non sequitur, pro rata, quid pro quo, sic, vice versa.