NOUN (3)

1. a seasonal wind in southern Asia; blows from the southwest (bringing rain) in summer and from the northeast in winter;

2. rainy season in southern Asia when the southwestern monsoon blows, bringing heavy rains;

3. any wind that changes direction with the seasons;

The Collaborative International Dictionary of English v.0.48:

monsoon \mon*soon"\ (m[o^]n*s[=oo]n"), n. [Malay m[=u]sim, fr.
   Ar. mausim a time, season: cf. F. monson, mousson, Sr.
   monzon, Pg. mon[,c][~a]o, It. monsone.]
   A wind blowing part of the year from one direction,
   alternating with a wind from the opposite direction; -- a
   term applied particularly to periodical winds of the Indian
   Ocean, which blow from the southwest from the latter part of
   May to the middle of September, and from the northeast from
   about the middle of October to the middle of December.
   [1913 Webster]

   2. A heavy rainfall in India associated with the southwest
      monsoon[1].
      [PJC]

   3. The season in which the monsoon[2] occurs.
      [PJC]

WordNet (r) 3.0 (2006):

monsoon
    n 1: a seasonal wind in southern Asia; blows from the southwest
         (bringing rain) in summer and from the northeast in winter
    2: rainy season in southern Asia when the southwestern monsoon
       blows, bringing heavy rains
    3: any wind that changes direction with the seasons

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "monsoon":
   air current, cat-and-dog weather, crosscurrent, current,
   current of air, dirty weather, downdraft, draft, fall wind,
   flow of air, following wind, head wind, indraft, inflow,
   inhalation, inrush, inspiration, jetstream, katabatic wind,
   movement of air, predominance of Aquarius, raininess, rains,
   rainy day, rainy season, rainy weather, spell of rain,
   spring rains, stormy weather, stream, stream of air, tail wind,
   undercurrent, updraft, wet, wet season, wet weather, wind