The Collaborative International Dictionary of English v.0.48:

Criminal \Crim"i*nal\ (kr?m"?-nal), a. [L. criminalis, fr.
   crimen: cf. F. criminel. See Crime.]
   1. Guilty of crime or sin.
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            The neglect of any of the relative duties renders us
            criminal in the sight of God.         --Rogers.
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   2. Involving a crime; of the nature of a crime; -- said of an
      act or of conduct; as, criminal carelessness.
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            Foppish and fantastic ornaments are only indications
            of vice, not criminal in themselves.  --Addison.
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   3. Relating to crime; -- opposed to civil; as, the criminal
      code.
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            The officers and servants of the crown, violating
            the personal liberty, or other right of the subject
            . . . were in some cases liable to criminal process.
                                                  --Hallam.
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   Criminal action (Law), an action or suit instituted to
      secure conviction and punishment for a crime.

   Criminal conversation (Law), unlawful intercourse with a
      married woman; adultery; -- usually abbreviated, crim.
      con.

   Criminal law, the law which relates to crimes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

criminal conversation
    n 1: extramarital sex that willfully and maliciously interferes
         with marriage relations; "adultery is often cited as
         grounds for divorce" [syn: adultery, criminal
         conversation, fornication]

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

CRIMINAL CONVERSATION, crim. law. This phrase is usually employed to denote
the crime of adultery. It is abbreviated crim. con. Bac. Ab. Marriage, E 2;
4 Blackf. R. 157.
     2. The remedy for criminal conversation is, by an action on the case
for damages. That the plaintiff connived, or assented to, his wife's
infidelity, or that he prostituted her for gain, is a complete answer to the
action. See Connivance. But the facts that the wife's character for chastity
was bad before the plaintiff married her; that he lived with her after he
knew of the criminal intimacy with the defendant; that he had connived at
her intimacy with other men;, or that the plaintiff had been false to his
wife, only go in mitigation of damages. 4 N. Hamp. R. 501.
     3. The wife cannot maintain an action for criminal conversation with
her husband; and for this, among other reasons, because her husband, who is
particeps criminis, must be joined with her as plaintiff.