Tribute, called Release (リリース Rirīsu) in the OCG and the Yu-Gi-Oh! 5D's and Yu-Gi-Oh! ZEXAL anime series (previously called Sacrifice (生け贄に捧げる Ikenie ni Sasageru) in the OCG and the Yu-Gi-Oh! and Yu-Gi-Oh! GX anime series, and Offer as a Tribute in the TCG) 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).