The Christian discipline of putting to death indwelling sin in the believer by the power of the Holy Spirit. From Latin mortificare, to put to death. The biblical anchors are Romans 8:13 (For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live) and Colossians 3:5-9 (Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry... put off all these; anger, wrath, malice, blasphemy, filthy communication out of your mouth. Lie not one to another, seeing that ye have put off the old man with his deeds). The Lord Jesus's own discipline-language is sharper: if thy right eye offend thee, pluck it out, and cast it from thee... if thy right hand offend thee, cut it off (Matthew 5:29-30). John Owen's The Mortification of Sin in Believers (1656) is the classical Reformed treatment, summarized in his famous principle: be killing sin, or it will be killing you. Owen's exposition identifies the discipline as Spirit-empowered (not bare moral effort), particular (against the specific sins the believer struggles with), gospel-centered (motivated by Christ's accomplished work and the believer's union with Him), sustained (not a one-time event but a lifelong labor), and integrated with the means of grace (the Word, prayer, the sacraments, fellowship). The patriarchal-Reformed reader holds mortification as the dark-and-light double-edged side of biblical sanctification: the believer puts to death indwelling sin and puts on the new man (vivification).