Definition from Wiktionary, the free dictionary
polygraph (plural polygraphs)
- 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.
- (dated) An mechanical instrument for multiplying copies of a writing, resembling multiple pantographs.
- (archaic) A collection of different works, either by one or several authors.
- (device recording physiological measurements): lie detector
- (dated instrument for copying): manifold writer, autopen
See also 
device to discern if a subject is lying
- To administer a polygraph test.
- The FBI polygraphed the suspect but learned nothing because they already knew he was lying.