Any one of the acalephs, esp. one of the larger species, having a jellylike appearance. See Medusa.
- Any of numerous usually free-swimming marine coelenterates of the class Scyphozoa, characteristically having a gelatinous, tentacled, often bell-shaped medusoid stage as the dominant phase of its life cycle.
- Any of various similar or related coelenterates.
- Informal One who lacks force of character; a weakling.