It's short for "cavalry scout". According to Wikipedia, "the Cavalry Scout is a job title and a reconnaissance specialist in the United States Army. Cavalry Scouts work to obtain, distribute and share vital combat and battlefield information on the enemy and on combat circumstances and environmental conditions. "

In the past it probably referred to a scouting unit on horseback. The word Calvary entailed any unit on horseback such as i know. However in modern days there are Air Cav, and Mechanized Cav referring to helicopters and armored vehicles