The Big Key is a recurring dungeon item in The Legend of Zelda series.
First appearing in A Link to the Past, the Big Key can be found in every dungeon of the game. It is used to open up certain doors with big locks, and the boss door. The big key can also be used to open the Big Chest containing the dungeon item.
Sprite from A Link to the Past
The Big Key from Four Swords Adventures
Obtaining a Big Key in The Minish Cap