How is psychosis related to depression?

Both brain disorders: They are two distinct mental / biochemical / neurotransmitter disorders. If depression become severe enough, psychosis can develop. The reverse is less common. Remember, i'm an internist, not a psychiatrist.

Depends: Psychosis is a group of symptoms that can occur in some cases of severe depression. These symptoms include hallucinations and delusions. In cases where psychosis is part of a depression it is called a psychotic depression.

Complex: Common depression has nothing to do with psychosis. But there are conditions where depression and psychosis co-occur. In depression with psychotic features, psychotic symptoms (e.g., hallucinations, delusions) are superimposed on major depression. In schizoaffective disorder, depression is superimposed on schizophrenic symptoms. Biploar mood disorder can include psychosis.