A severe form of depression resulting from a progressive brain disorder in which cognitive changes mimic those of dementia.

A depressive disorder is a syndrome (group of symptoms) that reflects a sad mood exceeding normal sadness or grief. More specifically, the sadness of depression is characterized by a greater intensity and duration and by more severe symptoms and functional disabilities than is normal.

Depression symptoms are characterized not only by negative thoughts, moods, and behaviors, but also by specific changes in bodily functions; for example, irregular eating, sleeping, crying spells, and decreased libido.

The functional changes of clinical depression are often called "neurovegetative signs". This means that the nervous system changes in the brain cause many physical results that result in diminished activity and participation.