Tetragrammaton
The Tetragrammaton (Greek, "four letters") is the unspeakable name of God in Hebrew Bible: YHWH (יהוה) — Yod, He, Vau, He. Strictly forbidden to pronounce in Jewish tradition; replaced in reading by Adonai (Lord). Reconstructions of pronunciation: Yahweh, Jehovah (corrupted form).
In esoteric magic
Used in Western ceremonial magic, the Tetragrammaton is invoked for power and protection. Each letter corresponds to an element (Yod=fire, He=water, Vau=air, He=earth) and to a Kabbalistic Sephirah.