Stacking

Stacking is a feature in the Metroid series, which allows Samus Aran to retain the previous effects of any beam, missile, and even suit, and use them in conjunction with the most recently collected upgrades. Super Metroid was the first installment to introduce stacking beams, and is the only Metroid game known to date where upgrades can be switched on or off, while Metroid Fusion is the first Metroid game to introduce stacking missiles.

Trivia

 * Like Metroid and Metroid II: Return of Samus, Metroid Prime and Metroid Prime 2: Echoes are the first two of the Metroid Prime trilogy where there is no stacking beams. And like Super Metroid, Metroid Prime 3: Corruption is the first of the trilogy to introduce stacking beams.