Monsoon

Monsoon (n.): a periodic wind especially in the Indian Ocean and southern Asia; the season of the southwest  monsoon in India and adjacent areas that is characterized by very heavy rainfall; rainfall that is associated with the monsoon.

Monsoons are generally thought of as Asia's version of the hurricane. Technically, however, they are a seasonal reversal of the wind, especially in Asia. In fact, "monsoon" is Arabic for "season." Before I cover why this seasonal reversal occurs, I need to tell you a few things about the wind.
The main driving force of the wind is the pressure gradient force, or PGF. This is the pressure area over a distance. Wind wants to blow towards low pressure, but is sometimes prevented by doing that by the Coriolis force and other forces. In this case, the Coriolis force does not hinder the wind that much.
During the winter, the air over Asia is cooler. That means not very much rising, which in turn means high pressure. Thus, the wind blows away from the continent, toward the Indian Ocean and low pressure. However, in summer the air rises more. This lowers the pressure over Asia, making the wind switch from blowing towards the ocean to blowing towards the land. That seasonal switching is the monsoon.
The other definition of a monsoon, very heavy rainfall, is caused by this seasonal switching. The ocean as well as the land is a lot warmer in the summer. This means that more water will evaporate, making the air more humid. This humid air is carried inland and rises, which creates clouds. These clouds form what we generally think of as the monsoon.