The Collaborative International Dictionary of English v.0.48:

Pentecostal \Pen`te*cos"tal\, a.
   Of or pertaining to Pentecost or to Whitsuntide.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pentecostal
    adj 1: of or relating to or characteristic of any of various
           Pentecostal religious bodies or their members
    2: of or relating to or occurring at Pentecost
    n 1: any member of a Pentecostal religious body [syn:
         Pentecostal, Pentecostalist]