Depression is a prolonged or deep emotional sensation of sadness, being "blue" or "down."  Depressive feelings such as discouragement or sadness are perfectly normal if they do not become too severe or last too long.  Depression becomes a clinical problem if a person's mood becomes too depressed or if the episode lasts more than two weeks.  If there is any question of severity of a person's depression, he or she should have an immediate evaluation by a mental health professional or a physician in order to rule out suicidal intent.