the personal name of the God of Israel: his own name, the name he gives himself, by which his people are to call on him and know him as the God who is with them.
This is not a title like "God" or "Lord" but a name, the one Israel's God claims as his own. Where the common word for God can be used of any deity, this name belongs to one God alone and is never given to another. It is the name spoken in worship, sworn by in oaths, and built into thousands of Israelite personal names (the "-iah" and "Jeho-" pieces in names like Isaiah, Jeremiah, and Jehoshaphat all carry it).
The name looks like a form of the old Hebrew verb "to be" or "to become," and the Bible plays on that connection at the burning bush, where God answers Moses with the words usually translated "I am who I am" or, as many now read them, "I will be who I will be" (Exodus 3:14). On that reading the name is less a statement about God's timeless existence than a promise of his presence: he will be there, with his people, as the one he chooses to be for them. The scholars who study the name caution that this is a meaningful play on its sound, not a dictionary definition of where the name came from, which is older than Israel's reflection on it.
Because the name carries the whole story of the covenant, it gathers up everything that story tells about this God: that he made the world, rescued Israel from Egypt, gave the law, keeps his promises, and tolerates no rival. To speak the name is to invoke that particular God and no other. That is why later Jewish reverence grew so careful around it, and why the same name stands at the heart of the call to worship him alone.