Unetanneh Tokef, Unethanneh Toqeph, or Unesanneh Tokef (ונתנה תוקף) is a piyyut that has been a part of the Rosh Hashanah and Yom Kippur liturgy in rabbinical Judaism for centuries. It introduces the Kedusha of Musaf for these days. Describing the important place this prayer has in the service, the ArtScroll machzor calls it "one of the most stirring compositions in the entire liturgy of the Days of Awe." It is probably the best-known piyyut after Kol Nidrei.
Read more about Unetanneh Tokef: Composition of Unetanneh Tokef, Position in The Prayer Service, Themes and Sources of Unetanneh Tokef, Popular Culture