The Greek noun eleusis means "coming" or "arrival" — the act of coming to a place. It appears once in the NT: Acts 7:52, where Stephen says the prophets "announced the coming (eleuseos) of the Righteous One." The word is related to erchomai (to come) and is virtually synonymous with parousia (presence/coming) in some contexts.
Eleusis (coming/advent) in Acts 7:52 summarizes Stephen's entire sermon: all the prophets pointed forward to one moment — the eleusis of the Righteous One (the Just One). This is the fulfillment of Israel's entire prophetic tradition. Yet Stephen's accusation is devastating: those who had the prophets then became the betrayers and murderers of the very One the prophets announced. The eleusis that Israel longed for was rejected when it arrived in person. The word thus captures both the hope of advent and the tragedy of human blindness.