In Revelation, we learn that the angels are singing "Holy, Holy, Holy is the Lord God almighty!" We speak of "holy men" and there might be a "holy moment". We strive for "personal holiness".
I've always had difficulty getting a grasp of this concept. Sometimes, it seems that "holiness" is used synonymously with "moral purity" or "sinlessness". I've also heard "being holy" equated with "being wholly other or set apart or different", but what's that supposed to mean?
As a first approximation, x is holy iff x is really, really morally good. But this feels like its lacking. Any ideas?