A leviathan is a giant sea creature. It can be real, like a whale, or mythical.
Moby Dick is an example of a famous leviathan. The word comes from Hebrew livyathan which means a great sea serpent or sea monster. A real leviathan is the giant sea squid Architeuthis, which was photographed alive for the first time in 2005. A leviathan can also be something that is really, really big. The Titanic was a leviathan that now rests with leviathans.