What is a period of heavy rain in India and Southeast Asia called?

A monsoon (from the Arabic "mausam," meaning "season") is a seasonal shift in the prevailing wind direction, that usually brings with it a different kind of weather. It almost always refers to the Asian monsoon, a large region extending from India to Southeast Asia where monsoon conditions prevail.

During the winter monsoon, a persistent and large high-pressure zone over Asia drives cool, dry air southward toward the tropics. This provides the monsoon region with its dry season.

Then during May and June of each year, the summer monsoon arrives with persistent southerly wind flow driven by a warm air mass with low pressure at the surface that forms over southern Asia as it is warmed by the sun. Air from the relatively higher pressure air mass over the Indian and tropical western Pacific Ocean flows northward toward the low pressure over land, bring with it torrential rains. A late arrival of the monsoon can be bad for agriculture, as the monsoon rains are necessary for summer crops.

In India, for example, the dry northerly wind flow over India changes direction, and warm humid air from the Indian Ocean flows from the south, gradually overspreading the Indian subcontinent. Widespread torrential rains and even severe thunderstorms, large hail and tornadoes can accompany the onset (arrival) of the summer monsoon.