Folie à deux

Folie à deux is a French phrase which means "madness shared by two". It is when two people both believe in something that isn't true and share their delusion together. It could be something like believing that aliens from outer-space have come to Earth, or that a person is in danger from another person. The people involved usually have a close relationship and spend a lot of time together, and one person's ideas can influence and infect the other person.