In legal terms, a plea is simply an answer to a claim made by someone in a criminal case under common law using the adversarial system. Colloquially, a plea has come to mean the assertion by a defendant at arraignment, or otherwise in response to a criminal charge, whether that person pleaded guilty, not guilty, no contest or (in the United States) 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."
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.