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The English popular name jellyfish has been in use since 1796.[4] It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as comb jellies) were included as "jellyfishes".[5] Even some scientists include the phylum ctenophora when they are referring to jellyfish.[6] Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column.[7]

As jellyfish are not true fish (which are vertebrates, unlike jellyfish), the word jellyfish is considered by some to be a misnomer. Public aquariums may use the terms jellies or sea jellies instead.[8] The term "jellies" may have become more popular than "jellyfish".[1] In scientific literature, "jelly" and "jellyfish" are often used interchangeably.[9] Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate.[10]

Many textbooks and sources refer to only scyphozoa as "true jellyfish".[11][12]

A group of jellyfish is sometimes called a bloom or a swarm.[13] "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected.[14] Another collective name for a group of jellyfish is a smack,[15] although this term is not commonly used by scientists who study jellyfish. Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance.[16] Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.[17]

Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs.[18]

The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology.[4] This term refers exclusively to the non-polyp life-stage which occurs in many cnidarians, which is typified by a large pulsating gelatinous bell with long trailing tentacles. All medusa-producing species belong to the sub-phylum Medusozoa.

In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the gastrovascular cavity and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles.