κῆτοςkētos, kay'-tos; probably from the base of G5490; a huge fish (as gaping for
prey):—whale.
Thayer's Greek Lexicon
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STRONGS G2785:
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.