The Collaborative International Dictionary of English v.0.48:

Res \Res\ (r?z), n.; pl. Res. [L.]
   A thing; the particular thing; a matter; a point.
   [1913 Webster]

   Res gestae [L., things done] (Law), the facts which form
      the environment of a litigated issue. --Wharton.

   Res judicata [L.] (Law), a thing adjudicated; a matter no
      longer open to controversy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

res judicata
    n 1: a matter already settled in court; cannot be raised again
         [syn: res judicata, res adjudicata]

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

RES JUDICATA, practice. The decision of a legal or equitable issue, by a
court of competent jurisdiction.
     2. It is a general principle that such decision is binding and
conclusive upon all other courts of concurrent power. This principle
pervades not only our own, but all other systems of jurisprudence, and has
become a rule of universal law, founded on the soundest policy. If,
therefore, Paul sue Peter to recover the amount due to him upon a bond and
on the trial the plaintiff fails to prove the due execution of the bond by
Peter, in consequence of which a verdict is rendered for the defendant, and
judgment is entered thereupon, this judgment, till reversed on error, is
conclusive upon the parties, and Paul cannot recover in a subsequent suit,
although he may then be able to prove the due execution of the bond by
Peter, and that the money is due to him, for, to use the language of the
civilians, res judicata facit ex albo nigrum, ex nigro album, ex curvo
redum, ex recto curvum.
     3. The constitution of the United States and the amendments to it
declare, that no fact, once tried by a jury, shall be otherwise reexaminable
in any court of the United States than according to the rules of the common
law. 3 Pet. 433; Dig. 44, 2; and Voet, Ibid; Kaime's Equity, vol. 2, p. 367;
1 Johns. Ch. R. 95; 2 M. R. 142; 3 M. R. 623; 4 M. R. 313, 456, 481; 5 M. R.
282, 465; 9 M. R. 38; 11 M. R. 607; 6 N. S. 292; 5 N. S. 664; 1 L. R. 318; 8
L. R. 187; 11 L. R. 517. Toullier, Droit Civil Francais, vol. 10, No. 65 to
259.
     4. But in order to make a matter res judicata there must be a
concurrence of the four conditions following, namely: 1. Identity in the
thing sued for. 2. Identity of the cause of action; if, for example, I have
claimed a right of way over Blackacre, and a final judgment has been
rendered against me, and afterwards I purchase Blackacre, this first
decision shall not be a bar to my recovery, when I sue as owner of the land,
and not for an easement over it, which I claimed as a right appurtenant to
My land Whiteacre. 3. Identity of persons and of parties to the action; this
rule is a necessary consequence of the rule of natural justice: ne inauditus
condemnetur. 4. Identity of the quality in the persons for or against whom
the claim is made; for example, an action by Peter to recover a horse, and a
final judgment against him, is no bar to an action by Peter, administrator
of Paul, to recover the same horse. Vide, Things adjudged.