Bomb (ボム Bomu) is Samus's down special move in all four installations of Super Smash Bros. series. Samus shifts into Morph Ball mode and lays a bomb, which will explode either after a set time period or upon contact with a character, shield, or breakable hitbox. If Samus is on top of a bomb when it explodes, she will enter Morph Ball mode and bounce up; thus, if Samus uses Bomb while touching an opponent, she will hit the opponent or their shield with the bomb and be popped up into the air. The mechanics behind the Morph Ball result in the Super Wavedash in Melee.
In Super Smash Bros, bombs deal 9% fresh and 7% stale.
In Melee, bombs almost always hit twice, although the two hits are essentially indistinguishable; if a bomb drops onto an opponent from above, it may hit them once and bounce off without exploding. They may do between 2% and 7% damage.
In Brawl, bombs also hit twice, and deal up to 9% fresh.
Bombing while in mid-air causes Samus to go into Morph Ball mode, and slows down her fall. Thus, in the Super Smash Bros. series, Samus can use the bomb recovery technique, which consists of repeatedly bomb while holding towards the stage to increase the distance of her recovery, although this takes away her midair jump if she still has not used it. In Melee, bombing twice consecutively in the air will cause Samus to pop upward from the first bomb, giving her near-infinite horizontal recovery. This is reputedly possible in Super Smash Bros. Brawl, but is less efficient.
Bombs may be used as part of Samus's approach in Melee. They can be used to pressure an opponent's shield and combo into other moves. Wavebombing is a technique that consists of dropping a bomb on an opponent followed immediately by a downwards airdodge: this functions as a quick wavedash and allows various continuations.
In Brawl, however, Samus is capable of also using her bombs to perform the Spider Ball technique. To perform, run off the edge of the stage or platform and immediately activate the bomb, then direct Samus back onto solid ground as she hops into the air. This will drop the bomb directly against the ledge of the stage, which may be used in edgeguarding.
Morph Ball bombs have been standard in all Metroid games, with the Morph Ball Bomb Jump, a common tool for puzzle solving. The appearance of the Morph Ball in all three Smash Bros. games is derived from the original Metroid, due to its lack of spiral patterns. In Brawl, it is also more detailed. Unlike Metroid games, the bombs are affected by gravity, that is they fall to the ground rather than staying in the air. In terms of the aforementioned bomb recovery, Samus in many Metroid games can use the explosion from a laid bomb to launch herself up, if she happens to be in Morph Ball mode that is. This usually is required for certain jumps into tight spaces only the Morph Ball mode can traverse through.
Drop bombs that explode after a few moments. Use the explosions to jump higher.
 Description from the Brawl Instruction Booklet
Drop a small timed bomb.