A witch is an individual who amasses more power than their natural reserve should allow them using spells and archaic tools to harness more power than the Cauldron has assigned them and use it to do good or evil.
According to Cassian, humans, and other creatures can become witches.
When Lord Devlon first sees Nesta Archeron, Feyre Archeron's older sister, he asks her if she is a witch, to which she replies that she is but only to make fun of him, not because she really is. Devlon thinks she is because of the aura of power she radiates which is because she had taken part of the Cauldron's power when it turned her into a High Fae.
- A Court of Silver Flames: Chapter 32