Kether, the Crown, is the first of the ten sefirot, the emanations through which, in Kabbalah, the infinite and unknowable divine, the Ein Sof, begins to manifest. It stands at the summit of the Tree of Life, above and beyond the reach of ordinary comprehension, the point at which the undifferentiated One first inclines toward creation; the later Kabbalists spoke of it as the ayin, the divine nothingness, so near to the source that it can barely be called a thing at all. Its divine name is Eheieh, "I Am", the name God gives at the burning bush in Exodus 3, "Ehyeh asher Ehyeh", I Am That I Am.
The ten sefirot are named as a group already in the Sefer Yetzirah, the Book of Formation, a terse cosmological text redacted somewhere between the third and sixth centuries, and elaborated into the full Tree by the medieval Kabbalah of the Zohar in thirteenth-century Spain and the systematisers who followed. Kether is pure being, the crown that is worn above the head rather than on it, the first and highest emanation.
Sources The ZoharTraditionally attributed to Moses de Leon with Sefer Yetzirah: The Book of Creation in Theory and PracticeAryeh Kaplan