Geburah, Severity or strength, is the fifth sefirah, the counterweight to Chesed and the necessary discipline of the Tree. It heads the left-hand Pillar of Severity beneath Binah and expresses judgement, restraint, power and the setting of limits, the divine severity that prunes, restricts and destroys what must not continue, so that mercy's boundless outflow is given form and justice. It is also called Din, judgement, or Pachad, fear.
In the Kabbalistic understanding Geburah is not evil but essential: without the cutting-back of severity the generosity of Chesed would run to formlessness, and the Kabbalists located the origin of evil precisely in severity unbalanced, judgement divorced from mercy. Its divine name is Elohim Gibor, the God of might or of battles. The sefirah takes its place in the doctrine named in the Sefer Yetzirah and developed across the medieval and early-modern Kabbalah; Geburah is the martial, disciplinary power of the divine, the strong hand and the sword, held in tension with the open hand of Chesed across the central axis of the Tree.
Sources The ZoharTraditionally attributed to Moses de Leon with Sefer Yetzirah: The Book of Creation in Theory and PracticeAryeh Kaplan