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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MANDAMUS, practice. The name of a writ, the principal word of which when the
proceedings were in Latin, was mandamus, we command.
     2. It is a command issuing in the name of the sovereign authority from
a superior court having jurisdiction, and is directed to some person,
corporation, or, inferior court, within the jurisdiction of such superior
court, requiring them to do some particular thing therein specified, which
appertains to their office and duty, and which the superior court has
previously determined, or at least supposes to be consonant to right and
justice. 20 Pick. 484; 21 Pick. 258; Dudley, 37; 4 Humph. 437.
     3. Mandamus is not a writ of right, it is not consequently granted of
course, but only at the discretion of the court to whom the application for
it is made; and this discretion is not exercised in favor of the applicant,
unless some just and useful purpose may be answered by the writ. 2 T. R.
385; 1 Cowen's R. 501; 11 Shepl. 151; 1 Pike, 11.
     4. This writ was introduced to prevent disorders from a failure of
justice; therefore it ought to be used upon all occasions where the law has
established no specific remedy, and where in justice and good government
there ought to be one. 3 Burr. R. 1267; 1 T. R. 148, 9.; 2 Pick. 414; 4
Pick. 68; 10 Pick. 235, 244; 7 Mass; 340; 3 Binn. 273; 5 Halst. 57; Cooke,
160; 1 Wend. 318; 5 Pet. 190; 1 Caines, R. 511; John. Cas. 181; 12 Wend.
183; 8 Pet. 291; 12 Pet. 524; 2 Penning. 1024; Hardin, 172; 7 Wheat. 534; 5
Watts. 152; 2 H. & M. 132; 3 H. & M. 1; 1 S. & R. 473; 5 Binn. 87; 3 Conn.
243; 2 Virg. Cas. 499; 5 Call. 548. Mandamus will not lie where the law has
given another specific remedy. 1 Wend. 318; 10 John. 484; 1 Cow. 417;
Coleman, 117; 1 Pet. 567; 2 Cowen, 444; 2 McCord, 170; Minor, 46; 2 Leigh,
165; Const. Rep. 165, 175, 703.
     5. The 13th section of the act of congress of September, 24, 1789,
gives the supreme court power to issue writs of mandamus in cases warranted
by the principles and usages of law, to any courts appointed or persons
holding office, under the authority of the United States. The issuing of a
mandamus to courts, is the exercise of an appellate jurisdiction, and,
therefore constitutionally vested in the supreme court; but a mandamus
directed to a public officer, belongs to original jurisdiction, and by the
constitution, the exercise of original jurisdiction by the supreme court is
restricted to certain specified cases, which do not comprehend a mandamus.
The latter clause of the above section, authorizing this writ to be issued
by the supreme court, to persons holding office under the authority of the
United States, is, therefore, not warranted by the constitution, and void. 1
Cranch, R. 175.
     6. The circuit courts of the United States may also issue writs of
mandamus, but their power in this particular, is confined exclusively to
those cases in which it may be necessary to the exercise of their
jurisdiction. 7 Cranch, R. 504; 8 Wheat. R. 598; 1 Paine's R. 453. Vide,
generally, 3 Bl. Com. 110; Com. Dig. h. t; Bac. Ab. h.t.; Vin. Ab. h.t.;
Selw. N. P. h.t.; Chit. Pr. h.t.; Serg. Const. Index, h.t.; Ang. on Corp.
Index, h.t.; 3 Chit. Bl. Com. 265 n. 7; 1 Kent. Com. 322; Dane's Ab. Index,
h.t.; 6 Watts & Serg. 386, 397; Bouv. Inst. Index, h.t.; and the article
"Courts of the United States."