A form of psychological warfare in which you do things to convince someone they are going crazy. Named after the 1940 British film Gaslight, in which a man tries to do just this to the heroine of the story.
An example would be to change a person's furniture around and tell them it's always been that way, or that their name is a different name, and then they check their ID card and it says the different name, and everyone who's in on the gaslighting must call her by this new name, etc. In the film this is possible because the woman's mother died and she is ill in the house, confined inside. But however it is achieved, the aim of gaslighting is to reinforce doubt on the person's own sense of judgement, to the point where they no longer trust their own mind, and resign themselves to the idea that they are insane.