Holiness is often defined (and, perhaps, more generally than in any other way) to be conformity to God's law; — including conformity of the heart or feelings, as well as of the outward action. To this definition, or to others stated with the same import, though, perhaps, with some variation of terms, we do not propose to object. Perhaps it would not be easy to give a better one.
There is a great difference, however, between holiness defined and holiness practiced; — between holiness, abstractly considered, and holiness in realization. If, therefore, it may be important to know in what holiness consists by definition, it is certainly not less so to know who is the actual possessor of it. The Hebrew word, which is translated holiness, involves, as one of its elements, the idea of being set apart to a sacred or religious purpose. The English term holiness, in its original import, means whole-ness, completeness. And this idea, when the subject is contemplated in a practical point of view, ought not to be lost sight of.
Accordingly, presenting the matter in a little different light from that in which it is usually presented, it would not be improper to say, that the holy man is one who is whole or complete in God. If every part of the life of the creature is filled up and completed with the life of God, then he is a whole or holy man, and not otherwise. A holy man, therefore, is one who freely surrenders himself to God, that he may receive everything from God in return; — so that, by means of a divine life, operating as a central principle at the seat or heart of his own nature, he is brought into entire harmony with God, and fully represents the divine conception or idea in faith, in knowledge, in love, in will, in harmonizing with providence, in everything. Holiness, therefore, considered practically, is the perfect restoration of the divine life in the soul.
— edited from A Treatise on Divine Union (1851) Part 7, Chapter 10 by Thomas Cogswell Upham. You can find more of his work at the blog, The Hidden Life, managed by Craig L Adams at: http://thomascupham.blogspot.com