The Collaborative International Dictionary of English v.0.48:

Dervish \Der"vish\, Dervise \Der"vise\, Dervis \Der"vis\, n.
   [Per. derw[=e]sch, fr. OPer. derew to beg, ask alms: cf. F.
   derviche.]
   1. A Turkish or Persian monk, especially one who professes
      extreme poverty and leads an austere life.
      [1913 Webster]

   2. One of the fanatical followers of the Mahdi, in the Sudan,
      in the 1880's.
      [Webster 1913 Suppl.]

   3. in modern times, a member of an ascetic Mohammedan sect
      notable for its devotional exercises, which include
      energetic chanting or shouting and rhythmic bodily
      movement, such as whirling, leading to a trance-like state
      or ecstasy. From these exercises the phrase whirling
      dervish is derived.
      [PJC]

   4. figuratively, a person who whirls or engages in frenzied
      activity reminiscent of the dervish[3] dancing.
      [PJC]

WordNet (r) 3.0 (2006):

dervish
    n 1: an ascetic Muslim monk; a member of an order noted for
         devotional exercises involving bodily movements

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "dervish":
   Albigensian, Black Muslim, Catharist, Franciscan, Mohammedan,
   Moslem, Motazilite, Muhammadan, Muslim, Mussulman, Sabbatarian,
   Sectary, Shia, Shiite, Sufi, Sunnite, Trappist, Wahabi, Waldensian,
   abdal, abstainer, anchorite, ascetic, bhikshu, fakir, flagellant,
   hajji, hermit, imam, mendicant, muezzin, mufti, mullah, murshid,
   puritan, qadi, sannyasi, santon, sheikh, yogi, yogin