Gaslighting

Gaslighting is a slang for "making someone question their own reality".
The word also describes a person ("gaslighter") who uses words and sometimes tricks to make others unsure of what is real and what is not real. This can be very upsetting. Gaslighting is only possible when the other person is a vulnerable person, for example when the gaslighter is a powerful person or if the person being gaslighted is fearful that not believing the gaslighter will bring them grief. Sometimes, people don't know they are gaslighting others.
The term "gaslighting" comes from a play that was first performed in 1938, and which was then made into a movie in 1944. The movie was called "Gaslight", and it was a story about a husband who tries to make his wife believe that she is going crazy by constantly dimming the gas lights in their home, while telling her that they were just as bright as usual. 
Nowadays gaslighting is used to describe a type of psychological abuse where the abuser makes their victim doubt their own sanity. This manipulative abuse ranges from subtle tactics such as contradicting everything the victim says, to more aggressive tactics such as outright lies. The result is a total destruction of the victim's trust in their own ability to recognise reality.