The Collaborative International Dictionary of English v.0.48:

Pentecost \Pen"te*cost\, n. [L. pentecoste, Gr. ? (sc. ?) the
   fiftieth day, Pentecost, fr. ? fiftieth, fr. ? fifty, fr. ?
   five. See Five, and cf. Pingster.]
   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]

WordNet (r) 3.0 (2006):

Pentecost
    n 1: seventh Sunday after Easter; commemorates the emanation of
         the Holy Spirit to the Apostles; a quarter day in Scotland
         [syn: Pentecost, Whitsunday]
    2: (Judaism) Jewish holy day celebrated on the sixth of Sivan to
       celebrate Moses receiving the Ten Commandments [syn:
       Shavous, Shabuoth, Shavuoth, Shavuot, Pentecost,
       Feast of Weeks]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "Pentecost":
   Advent, Allhallowmas, Allhallows, Allhallowtide, Annunciation,
   Annunciation Day, Ascension Day, Ash Wednesday, Candlemas,
   Candlemas Day, Carnival, Christmas, Corpus Christi,
   Day of Atonement, Easter, Easter Monday, Easter Saturday,
   Easter Sunday, Eastertide, Ember days, Epiphany, Fast of Av,
   Feast of Tabernacles, Feast of Weeks, Good Friday, Halloween,
   Hallowmas, Hanukkah, High Holy Days, Holy Thursday, Holy Week,
   Lady Day, Lammas, Lammas Day, Lammastide, Lent, Lententide,
   Mardi Gras, Martinmas, Maundy Thursday, Michaelmas, Michaelmas Day,
   Michaelmastide, New Year, Ninth of Av, Palm Sunday, Pancake Day,
   Passion Week, Passover, Pesach, Purim, Quadragesima,
   Quadragesima Sunday, Rosh Hashanah, Septuagesima, Shabuoth,
   Shrove Tuesday, Simhath Torah, Sukkoth, Trinity Sunday,
   Twelfth-day, Twelfth-tide, Whit-Tuesday, White Sunday, Whitmonday,
   Whitsun, Whitsunday, Whitsuntide, Whitweek, Yom Kippur



Easton's 1897 Bible Dictionary:

Pentecost
   i.e., "fiftieth", found only in the New Testament (Acts 2:1;
   20:16; 1 Cor. 16:8). The festival so named is first spoken of in
   Ex. 23:16 as "the feast of harvest," and again in Ex. 34:22 as
   "the day of the firstfruits" (Num. 28:26). From the sixteenth of
   the month of Nisan (the second day of the Passover), seven
   complete weeks, i.e., forty-nine days, were to be reckoned, and
   this feast was held on the fiftieth day. The manner in which it
   was to be kept is described in Lev. 23:15-19; Num. 28:27-29.
   Besides the sacrifices prescribed for the occasion, every one
   was to bring to the Lord his "tribute of a free-will offering"
   (Deut. 16:9-11). The purpose of this feast was to commemorate
   the completion of the grain harvest. Its distinguishing feature
   was the offering of "two leavened loaves" made from the new corn
   of the completed harvest, which, with two lambs, were waved
   before the Lord as a thank offering.

     The day of Pentecost is noted in the Christian Church as the
   day on which the Spirit descended upon the apostles, and on
   which, under Peter's preaching, so many thousands were converted
   in Jerusalem (Acts 2).


Hitchcock's Bible Names Dictionary (late 1800's):

Pentecost, fiftieth