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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 that person pleaded Guilty, Not Guilty, No Contest or Alford plea.
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 offences and the defendant may be sentenced immediately. This produces a system known under American law 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.
A "blind plea" is a guilty plea entered with no plea agreement in place. One defendant accused of illegally protesting nuclear power, when asked to enter his plea, stated, "I plead for the beauty that surrounds us"; this type of unorthodox plea is sometimes referred to as a "creative plea," and will usually be interpreted as a plea of not guilty. Likewise, standing mute and refusing to enter any plea at all will usually be interpreted as a not guilty plea; the Federal Rules of Criminal Procedure, for instance, state, "If a defendant refuses to enter a plea or if a defendant organization fails to appear, the court must enter a plea of not guilty."
Peremptory pleas 
These are pleas which claim that a case cannot proceed for some reason. They are so called because, rather than being an answer to the question of guilt or innocence, they are a claim that the matter of guilt or innocence should not be considered.
They are :
- autrefois convict - where the defendant has already been convicted of the charge (and thus cannot be tried again)
- autrefois acquit - where he has previously acquitted of the same charge (and hence cannot be tried again, under the doctrine of double jeopardy),
- plea of pardon - where he has been pardoned for the offence.
- special liability to repair a road or bridge - in English law, where a defendant local authority alleges that a private landowner was responsible for repairing a road or bridge
United States 
"Voluntary and intelligent" 
A defendant who enters a plea of guilty must do so, in the phraseology of a 1938 Supreme Court case, Johnson v. Zerbst, "knowingly, voluntarily and intelligently". The burden is on the prosecution to prove that all waivers of the defendant's rights complied with due process standards. Accordingly, in cases of all but the most minor offences, the court or the prosecution (depending upon local custom and the presiding judge's preference) will engage in a plea colloquy wherein they ask the defendant a series of rote questions about the defendant's knowledge of his rights and the voluntariness of the plea. Typically the hearing on the guilty plea is transcribed by a court reporter and the transcript is made a part of the permanent record of the case in order to preserve the conviction's validity from being challenged at some future time. "Voluntary" has been described as "an elusive term which has come to mean not induced by 'improper' inducements, such as bribing or physical violence, but not including the inducements normally associated with charge and sentence bargaining (except for inducements involving 'overcharging' by prosecutors)." "Intelligent" has been described as "also an elusive term, meaning that the defendant knows his rights, the nature of the charge to which he is pleading, and the consequences of his plea."
Virtually all jurisdictions hold that defense counsel need not discuss with defendants the collateral consequences of pleading guilty, such as consecutive sentencing or even treatment as an aggravating circumstance in an ongoing capital prosecution. However, the Supreme Court recognized an important exception in Padilla v. Kentucky (2010), in which the Court held that defense counsel is obligated to inform defendants of the potential immigration consequences of a guilty plea. Thus a defendant who is not advised of immigration consequences may have an ineffective assistance of counsel argument.
In the U.S. federal system, the court must also satisfy itself that there is a factual basis for the guilty plea. However, this safeguard may not be very effective, because the parties, having reached a plea agreement, may be reluctant to reveal any information that could disturb the agreement. When a plea agreement has been made, the judge's factual basis inquiry is usually perfunctory, and the standard for finding that the plea is factually based is very low.
Special pleas 
Other special pleas used in criminal cases include the plea of mental incompetence, challenging the jurisdiction of the court over the defendant's person, the plea in bar, attacking the jurisdiction of the court over the crime charged, and the plea in abatement, which is used to address procedural errors in bringing the charges against the defendant, not apparent on the "face" of the indictment or other charging instrument. Special pleas in federal criminal cases have been abolished, and defences formerly raised by special plea are now raised by motion to dismiss.
A conditional plea is one where the defendant pleads guilty to the offense, but specifically reserves the right to appeal certain aspects of the charges (for example, that the evidence was illegally obtained).
In United States v. Binion, malingering or feigning illness during a competency evaluation was held to be obstruction of justice and led to an enhanced sentence. Although the defendant had pleaded guilty, he was not awarded a reduction in sentence because the feigned illness was considered to mean that he was not accepting responsibility for his illegal behavior.
English law 
In the English system, a plea is regarded as voluntary if the defendant, properly advised as to the possible alternatives by his counsel, has the freedom in his own mind to choose the plea he will make.
Until 1772, if a defendant refused to plead guilty or not guilty, his trial was delayed from taking place, and he was tortured until he either died or entered a plea. This was changed to allow the judge to enter a plea of not guilty if the defendant refused to plead.
- Margareth Etienne (Summer, 2005), The Ethics of Cause Lawyering: An Empirical Examination of Criminal Defense Lawyers as Cause Lawyers, Vol. 95 (4), The Journal of Criminal Law and Criminology (1973-), pp. 1195–1260
- Hurst, John (August 10), A-plant protestors being freed, Los Angeles Times
- National Lawyers Guild, LA Chapter, Questions and Answers about Civil Disobedience and the Legal Process
- Rule 11, Federal Rules of Criminal Procedure
- McDonald, William F. (1986-1987), Judicial Supervision of the Guilty Plea Process: A Study of Six Jurisdictions 70, Judicature, p. 203
- Chin, Gabriel J.; Holmes, Richard W. Jr. (2001-2002), Effective Assistance of Counsel and the Consequences of Guilty Pleas 87, Cornell L. Rev., p. 697
- Jenia Iontcheva Turner (Winter, 2006), Judicial Participation in Plea Negotiations: A Comparative View 54 (1), The American Journal of Comparative Law, pp. 199–267
- "Behavior of the Defendant in a Competency-to-Stand-Trial Evaluation Becomes an Issue in Sentencing". Journal of the American Psychiatric Association. Retrieved 2007-10-10.
- A. Davis (1971), Sentences for Sale