The Collaborative International Dictionary of English v.0.48:

downbeat \downbeat\ n. (Music)
   the first beat of a musical measure (as the conductor's arm
   moves downward).
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

downbeat
    n 1: the first beat of a musical measure (as the conductor's arm
         moves downward)

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "downbeat":
   Cassandra-like, Cassandran, Cassandrian, arsis, bar beat, beat,
   bout, circle, circuit, course, cycle, cynical, defeatist, diastole,
   dismal, gloomy, negative, negativistic, nihilistic, offbeat,
   pessimist, pessimistic, pulsation, pulse, revolution, rotation,
   round, series, spell, systole, thesis, throb, turn, uncheerful,
   upbeat, wheel