A common misuse of the term "monsoon" is to refer to INDIVIDUAL thunderstorms as "monsoons" (example - "The east valley was pounded by monsoons this evening!"). The correct statement would be "The east valley was pounded by strong thunderstorms this evening!"
Seasonal wind shifts often bring a dramatic increase in moisture, and associated shower and thunderstorm activity, to the affected region. As the monsoon ends, and the winds shift again, the reverse occurs, with much drier air moving into the area.
The best example of a monsoon on Earth occurs over the Indian sub continent. During the months of April through October, a moist southwest wind brings heavy rains to this region...while a dry northeast wind is prevalent during the remainder of the year.
In North America, a similar situation occurs over much of Mexico. For example, in Acapulco, rainfall averages 51.8 inches during the months of June through October...while only 3.3 inches falls during the remainder of the year.
In the United States, Arizona and New Mexico are located on the northern fringe of the Mexican Monsoon. For most of the year, winds aloft over the southwest U.S. are west to northwest. During the summer, winds turn to a more south to southeast direction, importing moisture from the Pacific Ocean, the Gulf of California and the Gulf of Mexico.