<p><strong>Noun</strong></p><ul><li>any freshwater or marine coelenterate of the class Hydrozoa, including free-swimming or attached types, as the hydra, in which one developmental stage, either the polyp or medusa, is absent, and colonial types, as the Portuguese man-of-war, in which both medusa and polyp stages are present in a single colony.</li></ul><p><strong>Adjective</strong></p><ul><li>belonging or pertaining to the Hydrozoa.</li></ul>