From the Super Mario Wiki
Bomb is Samus's down special move in the Super Smash Bros. series, based upon the Morph Ball Bomb upgrade of the Metroid series. When used, Samus curls up into her Morph Ball form and drops a bomb from above that detonates after a few seconds or when it hits an item or opponent. If Samus is caught in the explosion, she is sent back into the air, once again in Morph Ball form. In Super Smash Bros. Melee and Super Smash Bros. Brawl, the bomb deals two hits.
In Super Smash Bros. for Nintendo 3DS / Wii U, Bomb has two custom variations that can be unlocked. The first custom version is Slip Bomb, which trips foes with the explosion and can meteor smash opponents in the air but deals less damage. The second custom move is Mega Bomb, which drops a larger bomb that takes longer to explode but deals more damage; in addition, only one can be on-screen at a time.