monsoon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A wind system that influences large climatic regions and reverses direction seasonally.
• noun A wind from the southwest or south that brings heavy rainfall to southern Asia in the summer.
• noun The rain that accompanies this wind.

from The Century Dictionary.

• noun A wind occurring in the alternation of the trade-winds in India and the north Indian ocean.
• noun Any of the winds that have annual alternations of direction and velocity, arising from differences of temperature between continents or islands and the surrounding ocean.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun A heavy rainfall in India associated with the southwest monsoon{1}.
• noun The season in which the monsoon{2} occurs.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of a number of winds associated with regions where most rain falls during a particular season.
• noun Tropical rainy season when the rain lasts for several months with few interruptions.
• noun The rains themselves.
• noun Entire meteorological systems with such characteristics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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