Gaslighting is a term used to describe a form of emotional abuse. The term has gained more prominence in recent years but is in reference to a play from the 1940’s where the wife character is told that she’s imaging it when her husband starts manipulating the gaslights in their home. She thinks that it’s getting darker but he reassures her that there is no issue, despite being the cause of the dimming lights.
Gaslighting refers to when a person deliberately lies to another in order to make them question their own mental health. Another example from literature is The Twits by Roald Dahl. In The Twits, Mr Twit gradually alters the height of Mrs Twit’s walking stick so that she thought that she was shrinking
There are multiple reasons why someone will lie to you even when it is demonstrably untrue. Not all of these instances are gaslighting but some of them are.