A witch was a female humanoid being known to wield magic, usually in order to possess authority over phyical reality and to control the behaviours of others. It is difficult to define "witch" since only two have ever been formally recognised in the whole history of Narnia, though there were strong rumours of others. Most information about witches, therefore, is merely based off these two: Jadis (the "White Witch") and the so-called "Lady of the Green Kirtle". Both of these witches were characterized by greed, unusually long lifespans, a tremendous desire for power, and ruthless use of violence. It is unknown whether all witches were as selfish and heartless as these two. No witches were ever confirmed to be human beings, though they often looked very similar to humans. Jadis, in particular, was known to be half-jinn, half-giant.
It is widely accepted historical fact that (particularly in the early eras of Narnia) there was a community of at least a small group of witches who lived west of the Wild Lands of the North, past the northern bourder of the Western Wild, who were known through legend as the "Northern Witches." The White Witch was a definite member of this group and the Lady of the Green Kirtle was assumed by Rilian and his Underland-journeying rescuers to be a member as well.