behemoth (noun, often attributive) Definition of behemoth 1: often capitalized, religion: a mighty animal described in Job 40:15–24 as an example of the power of God 2: something of monstrous size, power, or appearance a behemoth truck Synonyms & Antonyms for behemoth Synonyms colossus dinosaur dreadnought elephant giant Goliath jumbo leviathan mammoth mastodon monster titan…