what exactly is a leviathan

According to Hobbes, the Leviathan is the mortal God underneath an immortal God. It is the power and strength of all the people confired into this one thing. The people give up their right to all things for security which inables the Sovereign to use the Leviathan to instill fear in the group of people to maintain peace at home and defent against enemies abroad. The commonwealth is the body of the Leviathan.

The origins of the word Leviathan come from the bible, in Job, it's considered to be sort of a sea monster type thing.