A Final Smash (formerly called a Super Smash or Brawl Move by fans) is a special type of attack shown to debut in Super Smash Bros. Brawl. It is activated by touching the Smash Ball item, which then allows the user to activate an absurdly powerful "Final Smash" attack. Known FSs are a fire cyclone used by Mario, called the "Mario Finale", Link using the Triforce Slash, which involves rapidly slashing an opponent while trapping them between two triforce pieces, Kirby using his Chef ability to cook an enemy in a pot, and Pikachu using Volt Tackle, which turns it into a large electric ball which you can control for a period of time. Samus has also displayed a move with Final Smash-esque power (a monstrous laser cannon), but it is unknown if it is actually a Final Smash, or even a move in the first place.
It was speculated that the "Wario Waft" was a Final Smash, a recent update Smash Bros. site confirmed it as his Down-B.