sun·fish / ˈsənˌfish/ • n. (pl. same or -fishes) 1. a large deep-bodied marine fish (family Molidae) of warm seas, with tall dorsal and anal fins near the rear of the body and a very short tail. Its several species include the very large ocean sunfish (Mola mola), also known as mola mola. 2. a nest-building freshwater fish native to North America and popular in aquariums. The freshwater sunfish family (Centrarchidae) also includes sport fish such as the black basses, rock bass, bluegill, and crappies.