Definitions for "Aleinu"
(ah-LAY-noo) n. Aleinu. Closing prayer of the three daily services (popular since the 13th century).
(lit., 'it is incumbent upon us'). Prayer consisting of two passages (Aleinu and V'al Kein) and a follow up paragraph (the Al Tirah) recited at the conclusion of each of the three daily prayer services (see Siddur Tehillat Hashem p.84).
Closing prayer of every synagogue service, proclaiming God's sovereignty.