'hydroid' originates from New Latin, specifically the taxonomic use 'Hydroida/Hydroidea', where Greek 'hydōr' (ὕδωρ) meant 'water' and Greek 'eidos' (εἶδος) meant 'form' (the suffix '-oid' meaning 'resembling or having the form of').

'hydroid' was coined in Modern English in the 19th century from New Latin taxonomic terms such as 'Hydroida' or 'Hydroidea', themselves built from Greek roots 'hydōr' + 'eidos'. The term entered biological usage to name these water-dwelling organisms and their polyp stages.

Originally used for members of the taxon Hydroidea/Hydroida, the usage has come to denote specifically the polyp form or an individual/colony of these hydrozoans; the meaning narrowed from a formal taxonomic name to common biological usage.