The Collaborative International Dictionary of English v.0.48:

Dionysia \Di`o*ny"si*a\, n. pl. [L., fr. Gr. ?.] (Class. Antiq.)
   Any of the festivals held in honor of the Olympian god
   Dionysus. They correspond to the Roman Bacchanalia; the
   greater Dionysia were held at Athens in March or April, and
   were celebrated with elaborate performances of both tragedies
   and comedies.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Dionysia
    n 1: an orgiastic festival in ancient Greece in honor of
         Dionysus (= Bacchus) [syn: Dionysia, Bacchanalia]