/huy'dreuh zoh"euhn/ , n.
1. 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.
adj.
2. belonging or pertaining to the Hydrozoa.
[ 1875-80; Hydrozo ( a ) (see HYDRO- 1 , -ZOA) + -AN ]