A Big Key is a reappearing item found in The Legend of Zelda series. The key always refers to the key that is used to open the locked door to the primary boss of the dungeon Link is currently in, and is usually found by completing a challenging puzzle. The Big Key is always more fantastic looking than a standard, regular Key. There are a variety of different names for a Big Key, including Boss Keys and Knightmare Keys. Normally Link will just place the item in his pockets when he comes in possession of it though in The Legend of Zelda: Phantom Hourglass for the Nintendo DS, Link will carry it around until he comes across the door leading to the boss.
It can be speculated that the Magical Key from the original Legend of Zelda evolved into the Big Key.