A form of intimidation or psychological abuse, sometimes called Ambient Abuse where false information is presented to the victim, making them doubt their own memory, perception and quite often, their sanity. The classic example of gaslighting is to switch something around on someone that you know they're sure to notice, but then deny knowing anything about it, and to explain that they "must be imagining things" when they challenge these changes.
A more psychological definition of gaslighting is "an increasing frequency of systematically withholding factual information from, and/or providing false information to, the victim - having the gradual effect of making them anxious, confused, and less able to trust their own memory and perception.
Your spouse begins telling you things that never really happened. For instance, he says that last week he told you he was going to go to the bar with his buddies this Monday night, but you never remember him telling you that.
Or maybe your boss gets angry because you didn't prepare the report he asked you to. When you remind him that he usually prepares that particular report, he snaps that he told you to take care of it a few days ago because he was too busy. However, you know he never asked you to do so.
Source: Urban Dictionary
