Edward Shorter in Scientific American:
Major depression was created in 1980 by DSM-III editor Robert Spitzer as an effort to bridge disagreements between psychoanalysts, when they ruled the roost in the American Psychiatric Association, and the rest of the profession, which was becoming increasingly oriented towards biology. As a political construct, major depression included the two forms of depressive illness that previously had been considered as different from each other as measles and tuberculosis: melancholic illness and nonmelancholia. Melancholia, a grave form of depression involving slowed thought and movement, a complete joylessness in life and lack of hope for the future, had always been considered a separate illness. By 1980 the term melancholia had gone out of style and had been replaced by endogenous depression.
The other form of depressive illness that psychiatry had always recognized as separate was an ill-defined aggregation of symptoms of mood, anxiety, fatigue, somatic complaints – and a tendency to obsess about it all – that had been called on occasion neurasthenia, neurotic depression, reactive depression and other terms indicating real illness but not melancholic disease.
So the first artifact the DSM series created was lumping these two forms of depressive illness together. In fact, they are so disparate that the depression term itself should be abandoned.
More here. [Thanks to Louise Gordon.]