'cetacean' originates from New Latin, specifically the word 'Cetacea', where 'Cetacea' was formed from Latin 'cetus' (from Greek 'kētos') meaning 'sea monster or large sea creature'.

'cetacean' developed via New Latin 'Cetacea' and Medieval/Modern Latin 'cetaceus' derived from Latin 'cetus', which in turn came from Greek 'kētos' (κῆτος); these forms eventually yielded the English adjective/noun 'cetacean'.

Initially associated with the idea of a 'sea monster' or large sea creature, the term evolved to refer specifically to the scientific grouping of marine mammals now called whales, dolphins, and porpoises.