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 or pled guilty, not guilty, nolo contendere (a.k.a. no contest), no case to answer (in the United Kingdom), or Alford plea (in the United States).

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 defendant who pleads guilty is automatically convicted and the remainder of the trial is used to determine the sentence. This produces a system known as plea bargaining, in which defendants may plead guilty in exchange for a more lenient punishment. In civil law jurisdictions, 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 prosecutor from having to present a case to the court.