1.  pentecost(n = noun.time) whitsunday - seventh Sunday after Easter; commemorates the emanation of the Holy Spirit to the Apostles; a quarter day in Scotland;
is a kind of quarter day
is a part of whitsun, whitsuntide, whitweek
Derived form adjective pentecostal₂

2.  pentecost(n = noun.time) feast of weeks, shabuoth, shavous, shavuot, shavuoth - (Judaism) Jewish holy day celebrated on the sixth of Sivan to celebrate Moses receiving the Ten Commandments;
is a kind of jewish holy day
Derived form adjective pentecostal₂

1.  A solemn festival of the Jews; -- so called because celebrated on the fiftieth day (seven weeks) after the second day of the Passover (which fell on the sixteenth of the Jewish month Nisan); -- hence called, also, the Feast of Weeks. At this festival an offering of the first fruits of the harvest was made. By the Jews it was generally regarded as commemorative of the gift of the law on the fiftieth day after the departure from Egypt. [1913 Webster]

2.  A festival of the Roman Catholic and other churches in commemoration of the descent of the Holy Spirit on the apostles; which occurred on the day of Pentecost; -- called also Whitsunday. Shak. [1913 Webster]

pentecost, n.
1 a Whit Sunday. b a festival celebrating the descent of the Holy Spirit on Whit Sunday.
2 a the Jewish harvest festival, on the fiftieth day after the second day of Passover (Lev. 23:15-16). b a synagogue ceremony on the anniversary of the giving of the Law on Mount Sinai.

Etymology:
OE pentecosten & OF pentecoste, f. eccl.L pentecoste f. Gk pentekoste (hemera) fiftieth (day)

N  rite, ceremony, ritual, liturgy, ceremonial, ordinance, observance, function, duty, form, formulary, solemnity, sacrament, incantation, service, psalmody, ministration, preaching, preachment, predication, sermon, homily, lecture, discourse, pastoral, baptism, christening, chrism, circumcision, baptismal regeneration, font, confirmation, imposition of hands, laying on of hands, ordination, excommunication, Bar Mitzvah, Bas Mitzvah, Bris, Eucharist, Lord's supper, communion, the sacrament, the holy sacrament, celebration, high celebration, missa cantata, asperges, offertory, introit, consecration, consubstantiation, transubstantiation, real presence, elements, mass, high mass, low mass, dry mass, matrimony, burial, visitation of the sick, seven sacraments, impanation, subpanation, extreme unction, viaticum, invocation of saints, canonization, transfiguration, auricular confession, maceration, flagellation, sackcloth and ashes, penance, telling of beads, processional, thurification, incense, holy water, aspersion, relics, rosary, beads, reliquary, host, cross, rood, crucifix, pax, pyx, agnus Dei, censer, thurible, patera, eileton, Holy Grail, prayer machine, prayer wheel, Sangraal, urceus, ritualism, ceremonialism, sabbatism, sabbatarianism, ritualist, sabbatarian, holyday, feast, fast, Sabbath, Pentecost, Advent, Christmas, Epiphany, Lent, Passion week, Holy week, Easter, Easter Sunday, Whitsuntide, agape, Ascension Day, Candlemas, Ash Wednesday, Good Friday, Holy Thursday, Lammas, Martinmas, Michaelmas, All SAint's DAy, All Souls' Day, Ramadan, Ramazan, Bairam Passover, Shabuoth, Yom Kippur, Day of Atonement, Rosh Hashana, New Year, Hanukkah, Chanukkah, Feast of Lights, Purim, Feast of lots, ritual, ritualistic, ceremonial, baptismal, eucharistical, paschal, what art thou, thou idol ceremony?.