The term comes from a 1938 play, Gas Light, in which a man  starts to slowly drives his wife crazy by dimming the gas lighting in their house at random times while denying that anything is happening. She begins to mistrust her own experiences. After a while, she has no idea what to believe anymore.

Gaslighting is a form of psychological manipulation that seeks to sow seeds of doubt in a targeted individual or in members of a targeted group, making them question their own memory, perception, and sanity. Using persistent denial, misdirection, contradiction, and lying, it attempts to destabilize the target and delegitimize the target's belief.
Instances may range from the denial by an abuser that previous abusive incidents ever occurred up to the staging of bizarre events by the abuser with the intention of disorienting the victim. The term owes its origin to the 1938 Patrick Hamilton play Gas Light and its 1940 and 1944 film adaptations. The term has been used in clinical and research literature, as well as in political commentary.
Signs of gaslighting include:

• Trivializing the victim’s worth;
• Verbal abuse, usually in the form of jokes;
• Countering information to fit the abuser’s perspective;
• Withholding information from victim;
• Discounting information;
• Blocking and diverting the victim’s attention from outside sources;
• Undermining victim by gradually weakening them and their thought process.

See also:Manterrupting. Mansplaining, Broapriating

Source: https://en.wikipedia.org/wiki/Gaslighting