Specific criteria set out by the DSM, the most recent version of which is essentially a checklist of symptoms for doctors to classify mental health on behalf of insurance companies for billing and labeling purposes. As a layperson, undiagnosed and untreated for any 'mental illness', and in my personal opinion, every new edition of this 'psychologists bible' demonstrates to me the entire point is quickly getting lost, and serves a different master than the wellbeing of patients. Its confirmation bias for my decision to go into technology rather than psychology, so take this with a grain of salt.