The Greek word has both Jewish and Christian roots.
When referring to the Christian feast of Pentecost, it’s not immediately evident what the word itself means or where it came from. The name for the feast comes directly from the Acts of the Apostles where it reads, “When the day of Pentecost had come, they were all together in one place” (Acts 2:1).
Pentecost is a Greek word identifying the conclusion of the Jewish harvest festival. The Catholic Encyclopedia notes the other names this feast has in the Old Testament and why Pentecost was used by some Jews.
The term, adopted from the Greek-speaking Jews (Tob. 2:1; II Mac. 12:32; Josephus, “Ant.”, III, x, 6; etc.) alludes to the fact that the feast, known in the Old Testament as “the feast of harvest of the firstfruits” (Exodus 23:16), “the feast of weeks” (Exodus 34:22; Deuteronomy 16:10: 2 Chronicles 8:13), the “day of firstfruits” (Numbers 28:26), and called by later Jews ‘asereth or ‘asartha’ (solemn assembly, and probably “closing festival,” Pentecost being the closing festival of the harvest and of the Paschal season), fell on the fiftieth day from “the next day after the sabbath” of the Passover (Leviticus 23:11).
Leviticus explains how the celebration of this feast is to be counted, “And you shall count from the morrow after the sabbath, from the day that you brought the sheaf of the wave offering; seven full weeks shall they be, counting fifty days to the morrow after the seventh sabbath; then you shall present a cereal offering of new grain to the Lord” (Leviticus 23:15-16).
This is one term that carried over from Judaism to Christianity, as it also described the counting of 50 days before its celebration. The feast of Pentecost for Christians is held 50 days after Easter Sunday and marks the conclusion of the Easter season.