A galvanic skin response is a change in the ability of skin to conduct electricity. This is usually a result of an emotional stimulus, fright in most cases.
On the freighter Jovis, the door to the room where Data was held measured galvanic skin response as a method of biometric identification. (TNG: "The Most Toys")
When Geordi La Forge was brainwashed by Romulans in 2367 and beautiful images were directly sent to his visual cortex, his galvanic skin response dropped. When images of people suffering were fed to his visual cortex, however, his galvanic skin response increased by nine percent. (TNG: "The Mind's Eye")