Definition of Hydrozoa in English:
 Hydrozoa
plural noun ˌhʌɪdrə(ʊ)ˈzəʊəˌhīdrəˈzōə
Zoology A class of coelenterates which includes hydras and Portuguese men-of-war. Many of them are colonial and some kinds have both polypoid and medusoid phases.
 Example sentencesExamples
-  While the Cubozoa always have polyp and medusa stages, either one can be reduced in the Hydrozoa and Scyphozoa.
 -  The groups Hydrozoa, Scyphozoa and Cubozoa all are generally characterized as having polyps and medusae (although minor lineages have lost medusae and in some cases polyps).
 -  Traditional systematics divides the Hydrozoa into five orders.
 -  However, Hydra is not typical of the Hydrozoa as a whole.
 -  Another feature that is quite common in Hydrozoa but not typical of Scyphozoa is colonial organization.
 
Origin
  
Modern Latin (plural), from hydro- 'water' + Greek zōion 'animal'.