Source : Webster's Revised Unabridged Dictionary (1913)

Monsoon \Mon*soon"\, 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.

Source : WordNet®

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