Transmute is an ability that lets you discard the card with that ability from your hand in addition to paying its transmute cost. You may then search your library for a card with the same converted mana cost as the discarded card and put it in your hand.
Transmute was first introduced in Ravnica. It was an ability tied to the Dimir guild. It had mana costs ranging from 1 to 9 mana. It was later brought back in Future Sight where it got only one card, with a mana cost of 0.