In the Western world, Ancient History - the study of Antiquity - is generally thought to cover the period up to the fall of the Western Roman Empire (AD 476). Some other dates, earlier or later, are sometimes used to mark the end of Antiquity, but this one is the most common. In the academic study of the past, anything before that date is Ancient.

However, the word also just means "really old", and in informal usage it can mean anything you want it to.