hydromedusa Syllabification: (hy·dro·me·du·sa)Pronunciation: /ˌhīdrōməˈd(y)o͞osə, -zə/Definition of hydromedusa noun (plural hydromedusae /-ˌsē, -ˌzē/) Zoology the medusoid phase of a hydroid coelenterate. hydromedusa in other Oxford dictionaries Definition of hydromedusa in the British & World English dictionary