Mood disorder is a group of diagnoses in the Diagnostic and Statistical Manual of Mental Disorders classification system where a disturbance in the person’s mood is hypothesized to be the main underlying feature. The classification is known as mood (affective) disorders in International Classification of Diseases (ICD). English psychiatrist Henry Maudsley proposed an overarching category of affective disorder. The term was then replaced by mood disorder, as the latter term refers to the underlying or longitudinal emotional state, whereas the former refers to the external expression observed by others. Mood disorders fall into the basic groups of elevated mood such as mania or hypomania, depressed mood of which the best-known and most researched is major depressive disorder (MDD) (commonly called clinical depression, unipolar depression, or major depression), and moods which cycle between mania and depression known as bipolar disorder (BD) (formerly known as manic depression). There are several sub-types of depressive disorders or psychiatric syndromes featuring less severe symptoms such as dysthymic disorder (similar to but milder than MDD) and cyclothymic disorder (similar to but milder than BD). Mood disorders may also be substance-induced or occur in response to a medical condition.