Gaslighting describes actions that 1) make another person believe he or she is crazy, and 2) discredit the person by making others think they are crazy. The term comes from the play and 1944 movie Gaslight starring Charles Boyer and Ingrid Bergman. In this movie, the Charles Boyer character, acts as a primary aggressor. What he does is to manipulate the gas light in the house randomly from the attic. When the Ingrid Bergman character, his wife, reports this, he responds as though her perception is wrong. Because she has no explanation and because his manner is confident, she begins to doubt herself. It is not necessary to deliberately manipulate the environment to gaslight another person (although this happens).
Gaslighting happens mostly commonly when a survivor senses or perceives something about the primary aggressor that he does not not want to admit. When the survivors brings this up, the primary aggressor denies what the survivor is actually perceiving or sensing. This is more than just disagreeing--by tone and innuendo, or even outright, the perceptions are labeled improbable or crazy. Since information is incomplete, and since a sincere person is willing to contemplate being in error, the survivor begins to doubt herself. The primary aggressor also will emphasize and increase for a while those behaviors of his that are seemingly different. Also since the primary aggressor knows what is actually going on, there will be a certainty of manner, which while it is a lie, is still convincing. Over-time doubt can grow to the point at which the survivor doubts her sanity.
Being in relationship with someone with a hidden double life is also gaslighting. Because the primary aggressor has a contrary image to the rest of the world, the survivor knows that she or he will not be believed. This can be accentuated if the primary aggressor flaunts the secret behavior to the survivor, so that if she tells of the behavior to others, it will sound 'over-the-top' and therefore a fantasy. Others will be disbelieving and distance themselves from the survivor. The young women patients of Sigmund Freud who described their sexual abuse to him experienced this. The descriptions seemed so unlike the public image of the men responsible that Freud ultimately concluded it was fantasy, and he then took to telling the young women this. That is, he told them they were crazy!