Delusion SNOMEDCT_US:2073000 A delusion is a fixed false belief held despite evidence to the contrary. The term delusion broadly encompasses all false judgments that possess the following external characteristics to a significant, albeit unspecified, extent: (1) they are held with an exceptional level of conviction, accompanied by an unparalleled subjective certainty; (2) there is an inability to consider alternative experiences or compelling counter-arguments; (3) the content of the belief is impossible. HP:0000746 Delusions UMLS:C0011253 Disorder of thought content Abnormal judgment