Holiness to God is being like him. Several have posted this.
Therefore girding up the loins of your mind, being sober, perfectly hope for the grace being brought to you at the revelation of Jesus Christ, as obedient children, not fashioning yourselves according to the former lusts in your ignorance, but according to the Holy One who has called you, you also become holy in all conduct, because it is written, “
Be holy, for I am holy.” (
1 Peter 1:13-16)
How God is is how we are to be.
Same here:
48: Be ye therefore perfect,
even as your Father which is in heaven is perfect. Matt. 5:48
A simple definition is
BEING LIKE GOD.