"trade wind of the Indian Ocean," 1584, from Du. monssoen, from Port. monçao, from Ar. mawsim "appropriate season" (for a voyage, pilgrimage, etc.), from wasama "he marked." When it blows from the southwest (April through October) it brings heavy rain, hence "the rainy season" (1747).
A system of winds that influences the climate of a large area and that reverses direction with the seasons. Monsoons are caused primarily by the much greater annual variation in temperature over large areas of land than over large areas of adjacent ocean water. This variation causes an excess of atmospheric pressure over the continents in the winter, and a deficit in the summer. The disparity causes strong winds to blow between the ocean and the land, bringing heavy seasonal rainfall.
In southern Asia, a wind that is part of such a system and that blows from the southwest in the summer and usually brings heavy rains.