Folding is reactional magic related to folding space (but not time). It is taking two points apart from each other and bringing them together, either in a single moment such as teleporting or to create a persistent gate that can bridge two locations. It can also be used to create a pocket dimension, also known as a redoubt.
As magic goes, it has a large distance from almost every other sphere and is considered one of the rarest forms of magic. It also requires an incredible amount of energy to maintain which produces a significant amount of feedback for contrary resonances.
Despite pocket dimensions being a thing, there hasn't been enough mages capable of working with folding magic to discover an elemental to represent the dimension itself, which means all pocket dimensions must continually be maintained a reaction and will collapse as soon as the spell is disrupted.