The Tetragrammaton is the name YHWH, revealed to Moses. Since antiquity Jews have not pronounced it, reading Adonai, Lord, or ha-Shem, the Name, in its place. Kabbalah treats its letters as symbols of the sefirot and makes it the object of profound meditation.
Tetragrammaton
The four-letter name of God, YHWH, too holy to be pronounced in Jewish practice.
Also: YHWH, the Name