Cetacean is one of those words that never reached a wide use in the English language.
It is a general name you can use to indicate a whale, a dolphin or a porpoise. Yes, they are all cetaceans!
We call whales, dolphins and porpoises cetaceans because they all belong to the scientific order
Cetacea. This comes from the Latin Cetus, which meant a large sea animal.
In turn, the Latin word Cetus comes from the Greek Ketos (κῆτος actually, in Greek
alphabet), meaning sea monster.