Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

mandamus
     n : 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: {writ of
         mandamus}]