In some cases, the term "depression" will refer to full clinical depression. Other uses of the term may refer to the cluster of depressive disorders of which clinical depression is the most severe type. As mentioned above, the term "depression" may simply refer to depressive symptoms like sadness or down moods.

Diagnosis of depression is not always easy and an underlying cause is always possible. Depressive symptoms could be caused by an emotional upset (e.g. grief, divorce, job loss, etc), by drug abuse, or several other causes. Feeling "depressed" a few days after a major life crisis does not warrant a diagnosis of depression, and in fact taking antidepressants may be an inappropriate treatment in this case (e.g. they may avoid the healthy sequence of coping with grief or loss). However, over-diagnosis of depression and over-prescription of antidepressants in such situations is known to occur. Another possible over-diagnosis of depression is caused by the cycle of emotional symptoms that arise from premenstrual syndrome (PMS).