Transmute is a keyword ability.
It 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.
- 702.52a Transmute is an activated ability that functions only while the card with transmute is in a player’s hand. “Transmute [cost]” means “[Cost], Discard this card: Search your library for a card with the same converted mana cost as the discarded card, reveal that card, and put it into your hand. Then shuffle your library. Activate this ability only any time you could cast a sorcery.”
- 702.52b Although the transmute ability can be activated only if the card is in a player’s hand, it continues to exist while the object is on the battlefield and in all other zones. Therefore objects with transmute will be affected by effects that depend on objects having one or more activated abilities.