The Magic Mirror is an item that Link can come in possession of in the SNES video game The Legend of Zelda: A Link to the Past. He will obtain the item when he traverses through Death Mountain after helping an Old Man find his way back to his house. It's a crucial item in the game as it'll allow Link to go back and fourth from the light and Dark World.
It also possesses another unique function - if Link wishes to return back to the beginning of a dungeon he enters, then he can use the Magic Mirror to do so. This ability has been seen in almost every Zelda game following it, though to do so Link would have to find a different item (i.e. not the Magic Mirror).