any of various free-swimming, mostly marine cnidarians (esp. class Scyphozoa) with a body made up largely of jellylike substance and shaped like an umbrella: it has long, hanging tentacles with stinging cells on them
a. 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.
b. Any of various similar or related coelenterates.
Informal One who lacks force of character; a weakling.