1. (sea cow, mermaid) A herbivorous, fully aquatic, ungulate mammal (order Sirenia) that has a streamlined body, thick skin, limbs forming large paddles, and the tail modified into a horizontal tail fin. The order includes the dugongs and manatees.
2. A genus of permanently larval, aquatic salamanders (family Sirenidae) with long, eel-like bodies, short forelimbs, and no hindlimbs. There are two species found in the south-eastern USA.