The Big Key is a recurring 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.
Often times the Big Key can be found in Treasure Chests, sometimes found after defeating a dungeon Mini-Boss.
In later games, there have been slightly different variations of the Big Key, including the Nightmare Key from Link's Awakening and Boss Key from several Zelda titles.
Later games including The Wind Waker, The Minish Cap, and Twilight Princess contain the Big Key, though it serves essentially the same purpose as the Boss Key.