A monsoon is a seasonal wind system that reverses direction between summer and winter.
The reversal occurs mainly because land and water heat and cool at different rates. During a warm season, heated land can draw moist air from nearby oceans, producing widespread rainfall. During a cool season, land often becomes relatively colder, and winds can blow outward toward the sea, bringing drier conditions.
The best-known example is the South Asian monsoon, which strongly affects India, Bangladesh, Nepal, Pakistan, and surrounding regions. Its rains support agriculture but can also produce floods and landslides. Monsoons also occur in Africa, East Asia, Australia, and parts of the Americas.
A monsoon is not simply a single rainstorm or a synonym for heavy rain. It is a seasonal circulation pattern, and its rainfall varies from year to year because of ocean temperatures, snow cover, and other climate influences.