jellyfish Definition

: the typically free-swimming, bell-shaped, usually sexually-reproducing solitary or colonial form of a cnidarian in which the whorls of tentacles lined with nematocysts arise and hang down from the margin of the nearly transparent, gelatinous bell : medusa; especially : a large medusa characteristic of the siphonophores and scyphozoans (such as the sea nettle or box jellyfish)

siphonophore

ctenophore

any of various cube-shaped scyphozoan jellyfishes (order or suborder Cubomedusae) of tropical and warm marine waters that inflict a painful and sometimes fatal sting; especially : an extremely venomous box jellyfish (Chironex fleckeri) of Australia that has a bell that may reach over 11 inches (28 centimeters) in width with tentacles that may reach a length of over 78 inches (2 meters) : sea wasp

an extremely venomous box jellyfish (Chironex fleckeri) of Australia that has a bell that may reach over 11 inches (28 centimeters) in width with tentacles that may reach a length of over 78 inches (2 meters)