A person who refuses to answer questions or provide information under police
interrogation, while in
court, under
Congressional inquiry, or in similar situations because they believe that the answer would
incriminate them is said to be "pleading the fifth".
The term takes its name from the 5th Amendment to the Constitution of the United States of America, where in the following clause the right to do so is established:
"...nor shall [any person] be compelled in any criminal case to be a witness against himself..."
If a suspect has not been made aware of this right, or if a suspect has, against his will, been made to speak by use of torture, intimidation, or threats, a mistrial will often be declared, or a conviction overturned. To prevent this from happening, police officers usually make a point of making suspects aware of this right, most commonly in the "You have the right to remain silent. Anything you say can and will be used against you in a court of law..." portion of the "Miranda warning".
While for official purposes, the judicial system is to ignore an incidence of "pleading the fifth", acting as if the question were never asked, this does not in practice work quite as well. As a refusal to answer essentially means "a true answer would tend to indicate guilt on my behalf," often jurors, more rarely judges, and very commonly the media and the public interpret a refusal as an implicit admission of guilt.
For obvious reasons, the 5th Amendment is only recognized in the United States of America, although other countries may espouse similar protections. In spite of this, the American television ubiquity of the Fifth Amendment has made its protections internationally known, with multiple reports from non-American police and judicial systems of suspects attempting to take advantage of its protection.
The term also may refer to refusals to answer questions in other settings, much in the way that other terms like "giving him the third degree" or "reading them the Riot Act" have become distanced from their original judicial contexts.