Delusional disorder, previously called paranoid disorder, is a type of serious mental illness in which a person cannot separate the real from the imagined. The main feature of this disorder is the presence of delusions, unshakable beliefs in something untrue or not based on reality. People with delusional disorder also generally experience delusions involving situations that could occur in real life, such as being followed, poisoned, deceived, conspired against, or loved from a distance.