sanctification

Sanctification, in essence, is the process by which a person whose heart has been changed by the Holy Spirit is made more like Christ every day until His return or their promotion to heaven. The process is always the believers struggle with sin, empowered by the Holy Spirit. I have found John Owens Mortification of Sin to be a very good resource for understanding this. There is always going to be the conflict between the spirit and the flesh (Romans 7).

The phrase was coined as a catchcry for what is known as Keswick Theology which was formulated in the late 1800s in an attempt to spur the church onwards toward what they called a higher life.