From AMS Glossary
The belts of latitude over the oceans at approximately 30°–35°N and S where winds are predominantly calm or very light and weather is hot and dry.
These latitudes mark the normal axis of the subtropical highs, and move north and south by about 5° following the sun. The two calm belts are known as the calms of Cancer and calms of Capricorn in the Northern and Southern Hemispheres, respectively; in the North Atlantic Ocean, these are the latitudes of the Sargasso Sea. The name is believed to have originated in the days of sailing ships, when the voyage across the Atlantic in those latitudes was often prolonged by calms or baffling winds so that water ran short, and ships carrying horses to the West Indies found it necessary to throw the horses overboard.