Gaslighting
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.