The Collaborative International Dictionary of English v.0.48:

QED \QED.\ (k[=u]"[=e]*d[=e]"), n.
   Quantum electrodynamics. [abbrev.]
   [PJC] Q.E.D

The Collaborative International Dictionary of English v.0.48:

QED \Q.E.D\, QED \QED\(k[=u]"[=e]*d[=e]"), interj. [From Latin,
   quod erat demonstrandum, i.e. which was demonstrated.]
   Which was demonstrated; -- a phrase used after the conclusion
   of some line of reasoning, especially in mathematical or
   logical proofs. [abbrev.]
   [PJC]

WordNet (r) 3.0 (2006):

QED
    n 1: a relativistic quantum theory of the electromagnetic
         interactions of photons and electrons and muons [syn:
         quantum electrodynamics, QED]