(Latin: mortificare, to cause death)
A practise of Christian asceticism the purpose of which is twofold: negatively, to cause death to sin, to overcome the desires of the flesh, to conquer evil habits; positively, through penances, hardships, austerities and continued good actions, so to strengthen the will that a man may pursue a desired object despite difficulties.
A natural form of mortification is used by anyone who labors long hours to be successful; supernatural mortification aims at progress in virtue and the possession of God and depends on sanctifying grace. (Romans 8).
New Catholic Dictionary