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.
Gaslighting refers to the way, in which abusers lie to you, by word or deed, intentionally or not intentionally, to convince you that your version of reality is not right.
The phrase comes from the 1940's film Gaslight, in which an abusive husband deliberately dims the gaslights in the house, but when his wife comments on it he tells her she's imagining it, that the lights never dimmed at all.
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 mate 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
Gaslighting is one of the most insidious, viscious, nasty and effective forms of emotional and psychological abuse.
It can make the victim feel as if she's going crazy. If your perceptions of reality are constantly denied, and above all, denied by your partner, the person you love is very head-wrecking and crazy-making.
This gaslighting can be done deliberately, as in the example from the film above, in order to make you go crazy.
Another form of gaslighting is the denial of your right to be upset. In this case the abuser might accept that the situation happened, but will invalidate you by vehemently denying that there was anything untoward about it, or any valid reason to get upset.
You might also find the old classic movie Gaslight to be interesting. It clearly (and chillingly) shows exactly how gaslighting works.