Suffering from an ailment or disorder of the mind, e.g. Schizophrenia or Bipolar Disorder, usually treated by psychiatry. Some thinkers(eg Thomas Szasz, Peter Breggin) deny the existence of Mental Illness claiming it is a myth used to police cultural deviance. Many mentally ill people spend time wondering whether this perspective is true, and some conclude it is. This may be a reaction against society's unfair stigmatisation of people with psychiatric disorders, if there was no prejudice against sufferers (see psychotic for example) then perhaps more people would feel OK about owning up to having such an illness.
If the anti-psychiatry movement's basic premise is true then perhaps this would not be so surprising when one sees how other minority groups, e.g. racial ones, are stigmatised by a majority who wish to assert some sort of cultural hegemony.
Also to remember when looking at this definition is that symptoms of mental ill health like delusions, thought disorders of some kind or hallucinations occur in a spread through the entire "well" population, and thus no one is totally free from mental distress. There may conceivably be people who have received diagnoses of psychiatric problems who are objectively more sane than others who fate has not brought near a psychiatrist's office !