The Collaborative International Dictionary of English v.0.48:

Mandamus \Man*da"mus\, n. [L., we command, fr. mandare to
   command.] (Law)
   A writ issued by a superior court and directed to some
   inferior tribunal, or to some corporation or person
   exercising authority, commanding the performance of some
   specified duty.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mandamus
    n 1: an extraordinary writ commanding an official to perform a
         ministerial act that the law recognizes as an absolute duty
         and not a matter for the official's discretion; used only
         when all other judicial remedies fail [syn: mandamus,
         writ of mandamus]