The Collaborative International Dictionary of English v.0.48:

Mittimus \Mit"ti*mus\, n. [L., we send, fr. mittere to send.]
   (Law)
   (a) A precept or warrant granted by a justice for committing
       to prison a party charged with crime; a warrant of
       commitment to prison. --Burrill.
   (b) A writ for removing records from one court to another.
       --Brande & C.
       [1913 Webster]

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

MITTIMUS, English practice. A writ enclosing a record sent to be tried in a
county palatine; it derives its name from the Latin word mittimus, "we
send." It is the jury process of these counties, and commands the proper
officer of the county palatine to command the sheriff to summon the jury for
the trial of the cause, and to return the record, &c. 1 M. R. 278; 2 M. R.
88.



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

MITTIMUS, crim. law, practice. A precept in writing, under the hand and seal
of a justice of the peace, or other competent officer, directed to the
gaoler or keeper of a prison, commanding him to receive and safely keep, a
person charged with an offence therein named until he shall be delivered by
due course of law. Co. Litt. 590.