Tribute
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 from the field, either for a Tribute Summon, as a cost, or by an effect (that use this term).
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, excluding effects used on cards such as Stardust Dragon. This is one of the rare cases where tributing from the extra deck is acceptable.
A monster that requires a Tribute to Tribute Summon.
An example of a Ritual Spell Card that requires Tributes to Ritual Summon.