Clinicians sometimes use the word “psychotic" loosely. But it is better applied to specific kinds of abnormality: delusions and/or hallucinations. Hallucinations are generally defined as false sensory impressions, like hearing voices. Traditionally, auditory hallucinations are seen as occurring more in psychiatric illness, and visual hallucinations occur more in neurological illness; but both kinds can occur in both types of illness. Olfactory hallucinations tend to be neurological.
Delusions are more difficult to define. The traditional definition is a fixed, false belief, held against incontrovertible evidence to the contrary. In fact, delusions can be flexible (present some days, and not other days), true (the Othello syndrome: one has a delusion that one's spouse is cheating based on irrelevant reasoning, but in fact the spouse is cheating), and evidence can be quite strong against a belief despite its truth (e.g., the world seemed flat to almost everyone for a very long time). None of these features, by themselves, are sufficient to qualify for a definition of delusion. But together, the more such features are present (fixity, falsity, evidence against a belief), the more likely a delusion is present. Illogical reasoning is another feature that adds to the weight of delusionality.
Experimental psychology research has shown that all these features are present to some extent in normal non-psychotic individuals (fixity, falsity, illogicality). Delusions are extremes on the dimension of normal to abnormal thinking; they are not completely different psychological experiences from what is normal.
So what is psychosis? It’s an extreme of normal thinking where we view an individual as sufficiently out of touch with reality to define delusions or hallucinations. Many clinicians tend to assume that psychosis is an all-or-nothing phenomenon, contrasting with normality, which is completely diﬀerent. In fact, psychosis is a more-or-less phenomenon, representing more abnormal thinking than we find in non-psychotic persons. Since we are all a little abnormal in our thinking, we should be careful when diagnosis psychosis. Our belief that psychosis is present could be false.