noun. Any of various cnidarians of the class Hydrozoa, including the hydras, hydroids, and siphonophores, which exhibit alternation of generations between the polyp and medusa stage or exist in only one or the other stage.

Pertaining to the Hydrozoa; resembling the Hydrozoa, or having their characters.

noun. One of the Hydrozoa, as an acaleph, medusan, or jelly-fish.

noun. Any of many colonial coelenterates, of the class Hydrozoa, including the hydras, hydroids, hydrocorals, and siphonophores.

noun. colonial coelenterates having the polyp phase dominant

"Turritopsis Nutricula is technically known as a hydrozoan and is the only known animal that is capable of reverting completely to its younger self."

Hydrozoa