lie detector

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An instrument designed to detect when a person has answered a question falsely.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun informal A polygraph, a device which measures and records several physiological variables such as blood pressure, heart rate, respiration and skin conductivity while a series of questions is being asked to a subject, in an attempt to detect lies.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a polygraph that records bodily changes sometimes associated with lying