Jellyfish (also known as jellies or sea jellies) are free-swimming members of the phylum Cnidaria. They have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000-1500 species that make jellyfish and many more that do not). The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; medusa is another word for jellyfish. (Medusa is also the word for jellyfish in Modern Greek, Finnish, Portuguese, Romanian, Hebrew, Serbian, Croatian, Spanish, Italian, Hungarian, Polish, Lithuanian, Czech, Slovak, Russian, Bulgarian and Catalan).
Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water and are less than half an inch in size. They are partially white and clear and do not sting. This article focuses on scyphomedusae. These are the large, often colorful, jellyfish that are common in coastal zones worldwide.
In its broadest sense, the term jellyfish also generally refers to members of the phylum Ctenophora. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are generally transparent or translucent, and exist in shallow to deep portions of all the world's oceans.