Gaslighting is a form of psychological abuse. It involves an increasing frequency of systematically withholding factual information from, and/or providing false information to the subject, having the gradual effect of making the victim anxious, confused, and less able to trust his or her own memory and perception. A variation of gaslighting, used as a form of harassment, is to subtly alter aspects of a victim's environment, thereby upsetting his or her peace of mind, sense of security, etc.
The term was coined from the 1940 film Gaslight and its 1944 remake in which changes in gas light levels are experienced several times by the main character. The classic example in the film is the character Gregory using the gas lamps in the attic, causing the rest of the lamps in the house to dim slightly; when Paula comments on the lights' dimming, she is told she is imagining things. Paula believes herself alone in the house when the dimming occurs, unaware that Gregory has entered the attic from the house next door. The sinister interpretation of the change in light levels is part of a larger pattern of deception to which the character Paula is subjected.
This technique is also supposed to have been used by the Manson Family during their "creepy crawler" burglaries during which nothing was stolen, but furniture in the house was rearranged.
In the 2001 movie Amélie, the titular protagonist embarks on a mission to gaslight her local grocer as punishment for his cruelty toward his intellectually impaired assistant, Lucien. Amelie switches his lightbulbs with lower wattage bulbs and replaces his slippers with smaller ones, among other tricks.
In the 2007 movie The Darjeeling Limited, Adrian Brody's character asks Jason Schwartzman's character "Could she be gaslighting you?" when he discovers his ex-girlfriend had placed her perfume into his luggage.
The play The Mystery of Irma Vep makes reference to this with a scene where Lady Enid is recounting to her husband all of the strange things that have been happening in the house whereupon the lights begin dimming. When she comments on it, her husband assures her that the lights are not dimming, convincing her that she must be going crazy.
In Law & Order: Criminal Intent season 7, episode 22 (first aired August 24, 2008), Detective Eames (Kathryn Erbe) tells her partner Goren (Vincent D'Onofrio) that someone is "gaslighting" him. An as yet unidentified killer has arranged circumstances to bring suspicion upon and to unsettle Detective Goren in the murders of his brother (Tony Goldwyn) and his nemesis played by Olivia D'Abo.