- Any of numerous usually free-swimming marine cnidarians of the class Scyphozoa, characteristically having a gelatinous, tentacled, often bell-shaped medusa stage as the dominant phase of its life cycle.
- Any of various similar or related cnidarians.
- One who lacks force of character; a weakling.
- A popular name of many kinds of acalephs, medusas, sea-blubbers, or sea-nettles: so called from the soft, gelatinous structure.
- Any one of the acalephs, esp. one of the larger species, having a jellylike appearance. See <xref urlencoded="medusa">medusa</xref> and <xref urlencoded="acaleph">acaleph</xref>.
- Almost <xref>transparent</xref> aquatic being; any one of the <xref>acalephs</xref>, especially one of the larger species, having a jellylike appearance.
- any of numerous usually marine and free-swimming coelenterates that constitute the sexually reproductive forms of hydrozoans and scyphozoans
- large siphonophore having a bladderlike float and stinging tentacles