Goddess of Time

The Goddess of Time is a character mentioned in The Legend of Zelda: Majora's Mask. A great deity mentioned quite often by Tatl, it is revealed in a flashback that the goddess is responsible for Link's ability to travel back in time to the dawn of the First Day whenever he sees fit; when Link re-obtains his Ocarina of Time from the Skull Kid he remembers a meeting he shared with Princess Zelda, prior to leaving Hyrule. In this vision, Princess Zelda tells him that if he is ever in need of assistance, he should play the "Song of Time", and the Goddess of Time will come to his aid. It is also probable, though never explicitly stated, that the goddess also allows Link to manipulate time with the "Inverted Song of Time" and "Song of Double Time".

Theory
Many fans have come to believe that the Goddess of Time is in fact Nayru due to Princess Zelda's association with time as well as the Triforce of Wisdom.