early 15c., from Latin Titan, from Greek Titan, member of a mythological race of giants who attempted to scale heaven by piling Mount Pelion on Mount Ossa but were overthrown by Zeus and the gods. They descended from Titan, elder brother of Kronos. Perhaps from tito "sun, day," which is probably a loan-word from a language of Asia Minor. Sense of "person or thing of enormous size" first recorded 1828. Applied to planet Saturn's largest satellite in 1868; it was discovered 1655 by Dutch astronomer Christiaan Huygens, who named it Saturni Luna "moon of Saturn."
The gods in classical mythology who ruled the universe until they were overthrown by Zeus. Atlas and Prometheus were Titans.
Note: Any great and powerful person can be called a “titan.”