The key word, Monogenes, is drawn from the Gospel of John1:14, 1:18, and 3:16. The first of these verses describes the pre-incarnate Logos as being "only-begotten of the Father"; the second speaks of Jesus' earthly ministry; and the third describes the offering of the Incarnate Christ for the salvation of those who believe. The term Monogenes is also found in the Nicene Creed as established by the First Ecumenical Council in 325 AD.
References
^The Divine Liturgy of St. Basil the Great, Service Books of the Orthodox Church, II, South Canaan PA: St. Tikhon's Seminary Press, 1984, p. 38