From BR Bullpen
A or single A is the second lowest level of the minor leagues and has multiple levels of play: advanced full-season class A (ex. Carolina League), lower full-season class A (ex. South Atlantic League) and short-season class A (ex. New York-Penn League). The difference between the lowest class A and the highest is almost as large as the difference between AA and AAA; each league is made up of different types of players.
Some single A leagues: