Jellyfish

The word jellyfish (which has been in common usage for more than a century) is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). Some textbooks and websites refer to scyphozoans as "true jellyfish".
Since jellyfish are not even vertebrates, let alone true fish, the usual word jellyfish is considered by some to be a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead.
In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish. 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.
A group of jellyfish is sometimes called a bloom or a swarm. "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. Another collective name for a group of jellyfish is a smack.