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A mood is a relatively long lasting emotional state. Mood refers to a pervasive and sustained emotion. Mood is the underlying psychological, physiological and neurological driven emotional experience of the person. An individual’s mood is defined as "the sustained emotion that the individual is experiencing.” Moods differ from simple emotions in that they are more general, longer lasting, and pervasive.

Moods generally have either a positive or negative quality. In other words, people typically speak of being in a good mood or a bad mood. Unlike acute, affective emotional feelings, which are relatively brief, like fear and surprise, moods often last for hours or days. An individual’s mood critically affects perception and appraisal of the self and the environment. Mood is an internal, subjective state, but it often can be inferred from a person’s posture and other behaviors.

Affect expresses a subjectively experienced feeling state. Affect is the external expression of emotional responses to changing internal and external environmental conditions. A person’s affect is the external or behavioral expression of their internal state.

Mood disorder is the term given for a group of diagnoses where a disturbance in the person's mood is the main underlying feature. Long term disturbances of mood such as depression and bipolar disorder are considered mood disorders. Two groups of mood disorders are broadly recognized; the division is based on whether the person has ever had a manic or hypomanic episode. Thus, there are depressive disorders, of which the best known and most researched is major depressive disorder (MDD) commonly called clinical depression or major depression, and bipolar disorder (BD), formerly known as "manic depression" and experienced by intermittent periods of manic and depressed episodes.