κῆτος, κητεος (κήτους), τό, a sea-monster, whale, huge fish (Homer, Aristotle, others): Matthew 12:40, from Jonah 2:1 where the Sept., κήτειμεγάλῳ for גָּדולדַּג.
Matthew 12:40 - N-GNS GRK:κοιλίᾳ τοῦ κήτους τρεῖς ἡμέρας NAS: IN THE BELLY OF THE SEA MONSTER, so KJV: nights in the whale's belly; so INT: belly of the great fish three days