Depression is a word with many meanings — anything from a passing mood of sadness or discouragement to a condition of inconsolable misery, suicidal thoughts, and even delusions as well as severe physical symptoms. It's regarded as a clinical disorder when the depressed mood and related symptoms are serious enough or last long enough to interfere with work, social life, family life, or physical health.
Dysthymia, also known as persistent depressive disorder (PDD), is a mental disorder that produces the same cognitive and physical problems as depression, but with longer lasting symptoms. The concept was coined by Robert Spitzer as a replacement for the term "depressed personality" in the late 1970s.
In the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), dysthymia is a serious condition that causes chronic depression for at least two years (one year for children and adolescents). Dysthymia is less acute and more severe than major depressive disorder. Since dysthymia is a chronic troublesome disorder, people can experience it for many years before it is diagnosed. As a result, they may believe that depression is part of their character, so they can even discuss their symptoms with doctors, family members or friends. In DSM-5, dysthymia is replaced by a persistent depressed disorder. This new condition includes both chronic major depressive disorder and the previous dysthymic disorder. The change for the reason is that there are no significant differences between these two conditions.