The Collaborative International Dictionary of English v.0.48:

Sufism \Su"fism\, n.
   A refined mysticism among certain classes of Mohammedans,
   particularly in Persia, who hold to a kind of pantheism and
   practice extreme asceticism in their lives. [Written also
   sofism.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sufism
    n 1: Islamic mysticism