I came across a excellent quote on what holiness is by J. C. Ryle in doing some research the other day; I just had to share it.

"Holiness is the habit of being of one mind with God, according as we find His mind described in Scripture. It is the habit of agreeing in God’s judgment, hating what He hates, loving what He loves, and measuring everything in this world by the standard of His Word. He who most entirely agrees with God, he is the most holy man." - J. C. Ryle