The Pillar of Severity is the Tree of Life's left-hand column, running from Binah at the top through Geburah, whose own title translates as Severity or Strength, down to Hod, and it is read in Kabbalistic literature as the structural and symbolic counterweight to the Pillar of Mercy on the Tree's right side. Where the Pillar of Mercy carries an active, expansive quality, the Pillar of Severity is described as restrictive and form-giving, the force that limits, disciplines and gives definite shape to what the opposite pillar generates without bound, traditionally gendered feminine in older sources in a polarity scheme that pairs with, rather than simply mirrors, the masculine gendering of the Pillar of Mercy.
Geburah in particular carries a reputation within this pillar for severity in a harder sense as well, associated in some readings with judgment, warfare and necessary destruction, the pruning back that must accompany Chesed's unchecked growth on the opposite side of the diagram if the whole structure is not to become shapeless; this pairing of expansive mercy and restrictive severity, needing each other to produce a balanced whole rather than opposing each other as good against evil, is one of the Tree of Life's clearest built-in theological arguments about how the divine attributes work together.