Gaslighting involves a person who is the "mental abuser/victimizer" (the narcissist) and a second person, (you) the "victim". It is a severe form of manipulative psychological abuse carried out so subtly that the emotional abuse is not the type of in your face abuse but rather insidious. It is designed to make you doubt your perception, reality, and memory.
Gaslighting's sole purpose is to gain power over the victim. It's very dangerous and damaging; the narcissist tells blatant lies and spins the truth, often putting your safety and well being in danger.
Anyone is susceptible to gaslighting, and it is a common technique used by abusers, dictators, narcissists, and cult leaders. It is done slowly and methodically so the victim doesn't realize how much they've been brainwashed.
Gaslighting is used because the narcissist likes to cause confusion and make you doubt your sanity in order to control the conversation and confuse you. You will find yourself going around in argumentative circles both in your mind and out loud with the abuser, which can be physically exhausting and over time can damage your self trust and cause lower self esteem.
Gaslighting in a relationship can look like this: