The Collaborative International Dictionary of English v.0.48:

Flagellant \Flag"el*lant\, n. [L. flagellans, p. p. of
   flagellare: cf.F. flagellant. See Flagellate.] (Eccl.
   Hist.)
   One of a fanatical sect which flourished in Europe in the
   13th and 14th centuries, and maintained that flagellation was
   of equal virtue with baptism and the sacrament; -- called
   also disciplinant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flagellant
    n 1: a person who is whipped or whips himself for sexual
         gratification
    2: a person who whips himself as a religious penance