A Missile is a projectile found in many different video game series, notably the Metroid series of video games. Samus Aran is capable of ejecting missiles from her cannon when she has them. If she runs out, she'll be able to find more by destroying boxes and killing enemies. Some doors and opponents can only be opened or killed with missiles. In most Metroid games, a missile door took five missiles to open. In more recent titles, they only take one missile.
In the Super Smash Bros. series, Missile is her side B attack. If you smash attack it, the missile will go quickly striaght forward, though if you don't, it'll home on your opponent, though it will go much slower.