Catharsis is from the Ancient Greek word κάθαρσις, katharsis, meaning "purification" or "cleansing". In English it can refer to a number of different excreting acts.

It is most commonly used today to refer to the purification and purgation of thoughts and emotions, and to a resulting emotional state that results in renewal and restoration.

In dramaturgy, the term usually refers to arousing negative emotion in an audience, which then expels it, making them feel happier. In psychology, the term is associated with Freudian psychoanalysis where it specifically relates to the expression of buried trauma (the cause of a neurosis), bringing it into consciousness and releasing it, increasing happiness.

The term also has uses relating to the physical body. In medicine, it refers to the evacuation of the catamenia ("monthlies", menstrual fluid) from someone. Similarly, a cathartic is a substance that accelerates the defecation of faeces.

In Greek the term originally had a physical meaning only. This began with its use to describe purification practices. The first recorded use of the term being used in the mental sense was by Aristotle in the Politics and Poetics, comparing the effects of tragedy on the mind of a spectator to the effect of catharsis on the body.

The term is additionally used in Greek to refer to the spiritual purging process that occurs in the Christian Purgatory. Greek Neoplatonists also used the term to refer to spiritual purification.

Catharism was a term used by outsiders to describe the thinking of a European Christian group, so named because of its interest in purity.