similar in meaning to “holy” or “set apart.” It is the process whereby Christians are made holy through Christ's death, and continue to grow into and strive for holiness by cooperating with the indwelling of the Holy Spirit until they enjoy complete conformity to Christ (in heaven).
Literally, "being set apart" to God. The process of growth in Christ whereby the believer is made holy as God is holy, through the Holy Spirit (see article, "Deification," at 2 Pet. 1; Rom. 6:22 with center-column note; Rom. 15:16). See also DEIFICATION, JUSTIFICATION and SALVATION.
I like this definition from Thomas Watson's Sanctification. "It is a principle of grace savingly wrought, whereby the heart becomes holy, and is made after God's own heart. A sanctified person bears not only God's name, but his image." Also see Concise Theology chapter.
According to Wesleyan doctrine, a second (after being born again), definite, instantaneous work of grace which removes the inbred carnal nature through an act of the Holy Spirit, thereby making it less likely for the believer to sin again.
Sanctification or in its verb form, sanctify,literally means to set apart for special use or purpose, that is to make holy or sacred (compare Latin sanctus 'holy'). The Greek word is hagiasmos (Î¬Î³Î¹Î±ÏƒÎ¼Î¿Ï‚), meaning "holiness, consecration, or sanctification" (Verbrugge 9). It comes from the root hagios (Î¬Î³Î¹Î¿Ï‚), which means holy or sacred.