| n. pl. | 1. | (Zool.) The Acalephæ; one of the classes of cœlenterates, including the Hydroidea, Discophora, and Siphonophora. |
| Noun | 1. | Hydrozoa - coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes Synonyms: class Hydrozoa |