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]

The Collaborative International Dictionary of English v.0.48:

Whitsunday \Whit"sun*day\, n. [White + Sunday.]
   1. (Eccl.) The seventh Sunday, and the fiftieth day, after
      Easter; a festival of the church in commemoration of the
      descent of the Holy Spirit on the day of Pentecost;
      Pentecost; -- so called, it is said, because, in the
      primitive church, those who had been newly baptized
      appeared at church between Easter and Pentecost in white
      garments.
      [1913 Webster]

   2. (Scots Law) See the Note under Term, n., 12.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Whitsunday
    n 1: seventh Sunday after Easter; commemorates the emanation of
         the Holy Spirit to the Apostles; a quarter day in Scotland
         [syn: Pentecost, Whitsunday]