Why is the colosseum called the colosseum?

The Colossus of Rome was a statue of Nero outside of the amphitheatre. Roman citizens would refer to the area by that statue. "Let's go to the Coleseo (Colossus)" really referred to the amphitheatre and the statue which was immediately outside of the amphitheatre. Over time the amphitheatre came to be known as the Colosseum.