Source : Webster's Revised Unabridged Dictionary (1913)

Plea \Plea\, n. [OE. plee, plai, plait, fr. OF. plait, plaid,
   plet, LL. placitum judgment, decision, assembly, court, fr.
   L. placitum that which is pleasing, an opinion, sentiment,
   from placere to please. See {Please}, and cf. {Placit},
   {Plead}.]
   1. (Law) That which is alleged by a party in support of his
      cause; in a stricter sense, an allegation of fact in a
      cause, as distinguished from a demurrer; in a still more
      limited sense, and in modern practice, the defendant's
      answer to the plaintiff's declaration and demand. That
      which the plaintiff alleges in his declaration is answered
      and repelled or justified by the defendant's plea. In
      chancery practice, a plea is a special answer showing or
      relying upon one or more things as a cause why the suit
      should be either dismissed, delayed, or barred. In
      criminal practice, the plea is the defendant's formal
      answer to the indictment or information presented against
      him.

   2. (Law) A cause in court; a lawsuit; as, the Court of Common
      Pleas. See under {Common}.

            The Supreme Judicial Court shall have cognizance of
            pleas real, personal, and mixed.      --Laws of
                                                  Massachusetts.

   3. That which is alleged or pleaded, in defense or in
      justification; an excuse; an apology. ``Necessity, the
      tyrant's plea.'' --Milton.

            No plea must serve; 't is cruelty to spare.
                                                  --Denham.

   4. An urgent prayer or entreaty.

   {Pleas of the crown} (Eng. Law), criminal actions.

Source : WordNet®

plea
     n 1: a humble request for help from someone in authority [syn: {supplication}]
     2: (law) a defendant's answer by a factual matter (as
        distinguished from a demurrer)
     3: an answer indicating why a suit should be dismissed