Gennēsis (γέννησις) refers to the act or event of birth — a nativity. This word appears in Matthew 1:18 describing the birth of Jesus Christ and in Luke 1:14 in the context of John the Baptist's birth. It focuses specifically on the event of being born, in contrast to genesis which can encompass broader origin and lineage.
The birth (gennēsis) of Jesus in Matthew 1:18 is qualified immediately: "from the Holy Spirit" — making this the most unique birth in history. The incarnation is not merely the birth of a great man but the eternal Son of God taking on human flesh. The mystery of gennēsis here is that the Creator enters His own creation, the Ancient of Days is born of a woman (Galatians 4:4), and the Word becomes flesh (John 1:14). Heaven invades earth.