An actual sin that destroys sanctifying grace and causes the supernatural death of the soul. Mortal sin is a turning away from God because of a seriously inordinate adherence to creatures that causes grave injury to a person's rational nature and to the social order, and deprives the sinner of a right to heaven.
The terms mortal, deadly, grave, and serious applied to sin are synonyms, each with a slightly different implication. Mortal and deadly focus on the effects in the sinner, namely deprivation of the state of friendship with God; grave and serious refer to the importance of the matter in which a person offends God. But the Church never distinguishes among these terms as though they represented different kinds of sins. There is only one recognized correlative to mortal sin, and that is venial sin, which offends against God but does not cause the loss of one's state of grace. (Etym. Latin mors, death.)