What is "polyeleos"

The Polyeleos ( Greek: Πολυέλεος (pl. Πολυέλεοι), meaning "of much mercy", because of the repetition in one of the Polyeleoi of the phrase "ὅτι εἰς τὸν αἰῶνα τὸ ἔλεος αὐτοῦ", meaning "because forever [lasts] His mercy"), is a festive portion of the Matins or All-Night Vigil service as observed on higher-ranking feast days in the Eastern Orthodox and Byzantine Rite Catholic Churches. The Polyeleos is considered to be the high point of the service, and contains the reading of the Matins Gospel. Because of its liturgical importance, beautiful settings for the Polyeleos have been composed by Sergei Rachmaninoff and others.