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: Webster's Revised Unabridged Dictionary (1913)