Tribute (known as Release (リリース Rirīsu) in the OCG, Yu-Gi-Oh! 5D's and Yu-Gi-Oh! ZEXAL animes, previously known as Sacrifice in OCG, Yu-Gi-Oh! and Yu-Gi-Oh! GX animes) refers to the act results in sending a monster(s) the player controls to the Graveyard.
Monsters are Tributed in two ways:
Unless a card specifically states that you can Tribute monsters in your hand or under your opponent's possession, you can only Tribute monster you control on the field.