False beliefs (or judgements) that are held with extraordinary conviction and subjective certainty that are impervious to other experiences or counterarguments and the content is generally considered impossible for the context and circumstance. Delusional beliefs may arise meaningfully from personal or social experiences, yet are impossible, held with conviction, and irrefutable; more commonly they may be psychologically irreducible, appearing with no logical or meaningful connection to the individual experience. [from
HPO]