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 of 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 control, such as "Gishki Aquamirror" and "Reptilianne Vaskii", you can only Tribute monsters you control (on the field).