From Michaelis Dictionary:
plea
[pli:] n 1 argumento, pretexto, justificativa. 2 contenda, disputa. 3 apelo, rogo. 4 Jur objeção, contestação. on (ou under) the plea of sob o pretexto de. to make a plea 1 levantar uma objeção. 2 fazer um apelo. we put in a plea for fizemos um apelo em favor de.

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Plea
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Criminal procedure
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In legal terms, a plea is simply an answer to a claim made by someone in a civil or criminal case under common law using the adversary system. Colloquially, a plea has come to mean the assertion by a criminal defendant, at arraignment or otherwise in response to a criminal charge, whether he or she is Guilty or Not Guilty.

The concept of the plea is one of the major differences between criminal procedure under common law and procedure under the civil law system. Under common law, a plea of guilty by the defendant waives trial of the charged offenses and the defendant may be sentenced immediately. This produces a system under American law known as plea bargaining.

In civil law jurisdictions, there is generally no concept of a plea of guilty. A confession by the defendant is treated like any other piece of evidence, and a full confession does not prevent a full trial from occurring or relieve the plaintiff(s) from its duty of presenting a case to the trial court.

Eu imagino que "contra-alegação" seja uma resposta a uma alegação:

affirmative defense: affirmative defense n. when a defendant files an answer, in addition to denying some or all of the allegations, he/she can state what are called "affirmative defenses.
http://legal-dictionary.thefreedictionary.com/Affirmative De...

Provided new evidence is introduced, I think this is the term you are looking for. See the two glossaries and http://en.wikipedia.org/wiki/Rebuttal