Persistent belief in something which is not a fact is
Delusions are false fixed firm unshakable beliefs which are impervious to other experiences leads to compelling counter argument which are not in keeping with patient's socio-cultural and educational background. These are of two types: primary and secondary.1. Primary delusions arise de novo and cannot be explained on the basis of other experiences or perceptions. Also known as autochthonous delusions, these are thought to be characteristic of schizophrenia and are usually seen in early stages.2. Secondary delusions are the commonest type of delusions seen in clinical practice and are not diagnostic of schizophrenia as these can also be seen in other psychoses. Secondary delusions can be explained as arising from other abnormal experiences. Ref: A Sho Textbook of Psychiatry, Niraj Ahuja, 7th Edition, pg. no. 56
Illusion
Hallucination
Delusion
Delirium
Symptoms and signs in psychiatry and classification