holiness: belonging to God, and so claimed by him and kept apart from ordinary use
This is one of the richest words in the Hebrew Bible, and it grows out of a single idea: belonging to God. Whatever is holy has been given over to him, drawn into his sphere, and so it can no longer be treated as common (חֹל). The older textbook line that holy means basically "separate" is now widely questioned; the deeper sense is the positive one of being dedicated to God, with being set apart as the result that follows. The word can name the quality itself, above all God's own holiness, or it can name the concrete things that quality attaches to: a holy people, a holy place, a holy day, a holy gift. These are not unrelated meanings but one idea seen from many angles, which is why the same word covers them all.
The word's center is God himself. To say his name, his arm, or his throne is holy is to say they share his utter difference from everything created and his unrivaled majesty. In Isaiah this becomes a title, the Holy One of Israel, and there holiness is shown not only in dazzling glory but in justice and faithfulness toward his people. From God the quality radiates outward: because he is holy,
- the ground at the burning bush is holy
- the seventh day is holy
- the inner room of the tent is holy
- a people he chooses can be called holy too
Within Israel's worship the word does steady, practical work. The priest's basic job is to keep the line between what belongs to God and what is common, and between what is fit to approach him and what is not. To consecrate (קָדַשׁ) something moves it from common use into God's keeping; to profane it drags it back, which is a grave offense. So the holy is never to be used, touched, or eaten as if it were ordinary, and mishandling it is dangerous.
Scripture also shows holiness arriving in two different ways. Sometimes a thing or a people simply is holy because God has chosen it, an established status. Sometimes holiness is held out as a goal, something Israel is summoned to live into through obedience. The same word carries both, which is part of why it became the deepest theological term the tradition possessed.