Strictly speaking, hedonism is the belief that pursuing pleasure leads to the greatest ethical good. In practice, though, the ethical part sometimes gets lost in the pleasure part.
Hedonism originally hails from the Greek word hedone, meaning "pleasure." In Ancient Greece, hedonism really did have an ethical component, and its adherents really believed that society would be best served if you as an individual cultivated those things that gave you the greatest pleasure, instead of the self-denial inherent in other philosophies and religions. Over time the philosophical aspect of hedonism has faded and we are left an idea that looks to us very much like, well, Las Vegas.