Sanctification is a process in which God sanctifies us over time by grace. Yet the Bible speaks of sanctification in other ways, not relating to salvation, but in the 'setting apart' of one for a purpose or grace.
The WCF has a fine section detailing the Reformed view of sanctification.
http://www.reformed.org/documents/index.html?mainframe=westminster_conf_of_faith.html
I. They who are effectually called and regenerated, having a new heart and a new spirit created in them, are further sanctified, really and personally, through the virtue of Christ's death and resurrection, by his Word and Spirit dwelling in them; the dominion of the whole body of sin is destroyed, and the several lusts thereof are more and more weakened and mortified, and they more and more quickened and strengthened, in all saving graces, to the practice of true holiness, without which no man shall see the Lord.
II. This
sanctification is throughout in the whole man, yet imperfect in this life: there abideth still some remnants of corruption in every part, whence ariseth a continual and irreconcilable war, the flesh lusting against the Spirit, and the Spirit against the flesh.
III. In which war, although the remaining corruption for a time may much prevail, yet, through the continual supply of strength rom the sanctifying Spirit of Christ, the regerate part doth overcome: and so the saints grow in grace, perfecting holiness in the fear of God.
