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 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 on e 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.)