QED is a computer text editor.
Originally written by Butler Lampson and Peter Deutsch for the SDS 940 (in the 1960s?). Ken Thompson later wrote a version for CTSS; this version was notable for introducing regular expressions. QED influenced the classic UNIX text editor ed and the less well known sam.
History of QED
See also: Q.E.D.
QED is also a (sort of) widely used abbreviation of quantum electrodynamics