The Collaborative International Dictionary of English v.0.48:

Nolle prosequi \Nol"le pros"e*qui\ [L., to be unwilling to
   prosecute.] (Law)
   Will not prosecute; -- an entry on the record, denoting that
   a plaintiff discontinues his suit, or the attorney for the
   public a prosecution; either wholly, or as to some count, or
   as to some of several defendants.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nolle prosequi
    n 1: an entry in the court record to the effect that the
         plaintiff or prosecutor will not proceed [syn: nolle
         prosequi, nol pros]
    v 1: drop prosecution of by entering a nolle prosequi in the
         court records; "They nolle prossed the charge" [syn: nolle
         pros, nolle prosequi, nol.pros.]

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

NOLLE PROSEQUI, practice. An entry made on the record, by which the
prosecutor or plaintiff declares that he will proceed no further.
     2. A nolle prosequi may be entered either in a criminal or a civil
case. In criminal cases, a nolle prosequi may be entered at any time before
the finding of the grand jury, by the attorney general, and generally after
a true bill has been found; in Pennsylvania, in consequence of a statutory
provision, no nolle prosequi can be entered after a bill has been found,
without leave of the court, except in cases of assault and battery,
fornication and bastardy, on agreement between the parties, or in
prosecutions for keeping tippling houses. Act of April 29, 1819, s. 4, 7
Smith's Laws, 227.
     3. A nolle prosequi may be entered as to one ot several defendants. 11
East, R. 307.
     4. The effect of a nolle prosequi, when obtained, is to put the
defendant without day, but it does not operate as an acquittal; for he may
be afterwards reindicted, and even upon the same indictment, fresh process
may be awarded. 6 Mod. 261; 1 Salk. 59; Com. Dig. Indictment. K; 2 Mass. R.
172.
     5. In civil cases, a nolle prosequi is considered, not to be of the
nature of a retraxit or release, as was formerly supposed, but an agreement
only, not to proceed either against some of the defendants, or as to part of
the suit. Vide 1 Saund. 207, note 2, and the authorities there cited. 1
Chit. PI. 546. A nolle prosequi is now held to be no bar to a future action
for the same cause, except in those cases where, from the nature of the
action, judgment and execution against one, is a satisfaction of all the
damages sustained by the plaintiff. 3 T. R. 511; 1 Wils. 98.
     6. In civil cases, a nolle prosequi may be entered as to one of several
counts; 7 Wend. 301; or to one of several defendants; 1 Pet. R. 80; as in
the case of a joint contract, where one of two defendants pleads infancy,
the plaintiff may enter a nolle prosequi, as to him, and proceed against the
other. 1 Pick. 500. See, generally, 1 Pet. R. 74; see 2 Rawle, 334; 1 Bibb,
337; 4 Bibb, 887, 454; 3 Cowen, 374; 5 Gill & John. 489; 5 Wend. 224; 20
John. 126; 3 Cowen, 335; 12 Wend. 110; 3 Watts, 460.