A monsoon is defined as a seasonal reversing wind accompanied by corresponding changes in precipitation. In other words, monsoons are winds that change with the seasons, blowing easterly in the winter, and westerly in the summer.
A monsoon is caused by the movement northward from winter to summer of the huge upper level subtropical high pressure system, specifically known as the Bermuda High, and the intense heating of the Mohave Desert creating rising air and surface low pressure, called a thermal low. The Bermuda High is full of extremely cold air. This meeting with intense hot air in the surrounding air can cause low pressure. In the thermal low, desert areas cause intense solar-heating creating hot air. This hot air is less dense than the surrounding cool air, so the heat rises causing extreme low pressure. Low pressure always means stormy weather, so an intense amount of low pressure can cause a huge storm, a monsoon.