The Collaborative International Dictionary of English v.0.48:

Protestant \Prot"es*tant\, a. [Cf. F. protestant.]
   1. Making a protest; protesting.
      [1913 Webster]

   2. Of or pertaining to the faith and practice of those
      Christians who reject the authority of the Roman Catholic
      Church; as, Protestant writers.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Protestant \Prot"es*tant\, n. [F. protestant, fr. L. protestans,
   -antis, p. pr. of protestare. See Protest, v.]
   One who protests; -- originally applied to those who adhered
   to Luther, and protested against, or made a solemn
   declaration of dissent from, a decree of the Emperor Charles
   V. and the Diet of Spires, in 1529, against the Reformers,
   and appealed to a general council; -- now used in a popular
   sense to designate any Christian who does not belong to the
   Roman Catholic or the Greek Church.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Protestant
    adj 1: of or relating to Protestants or Protestantism;
           "Protestant churches"; "a Protestant denomination"
    2: protesting
    n 1: an adherent of Protestantism
    2: the Protestant churches and denominations collectively [syn:
       Protestant Church, Protestant]