an (imaginary or real) aquatic animal of enormous size; anything of giant size [n -S]
anything of immense size and power.
is a word with literary flair that can refer broadly to something very large and powerful, or more narrowly to a large sea animal, or a totalitarian state having a vast bureaucracy.
is something large or formidable.
often capitalized Leviathan a sea monster defeated by Yahweh in various scriptural accounts.