The Collaborative International Dictionary of English v.0.48:

QCD \QCD\ (k[=u]"s[=e]*d[=e]"), n. (Physics)
   Quantum chromodynamics. [abbrev.]
   [PJC]

WordNet (r) 3.0 (2006):

QCD
    n 1: a theory of strong interactions between elementary
         particles (including the interaction that binds protons and
         neutrons in the nucleus); it assumes that strongly
         interacting particles (hadrons) are made of quarks and that
         gluons bind the quarks together [syn: quantum
         chromodynamics, QCD]