Pen·te·cost 
(pĕntĭ-kôst′, -kŏst′)
(pĕntĭ-kôst′, -kŏst′)n.
1. Christianity The seventh Sunday after Easter, commemorating the descent of the Holy Spirit upon the disciples. Also called Whitsunday.
2. Judaism See Shavuot.
[Middle English pentecoste, from Old English Pentecosten, from Late Latin Pentēcostē, from Greek pentēkostē (hēmerā), fiftieth (day), feminine of pentēkostos, fiftieth, from pentēkonta, fifty; see penkwe in the Appendix of Indo-European roots.]