Monsoon is the term for the wind that carries heavy rains to southern Asia, and the rains themselves. During a monsoon, people wait indoors for it to stop.
Although people in other parts of the world sometimes describe heavy rainfall as a monsoon, the real thing only occurs in Asia. Monsoon describes both the rain that drenches India and Southeast Asia in summer and winter, and the wind that carries that rain. The rains are so heavy that they can lead to floods that wash away entire towns.