The simplest definition of a Witch is someone who practices Witchcraft. Anyone can identify as one, but if they do not practice Witchcraft, are they a Witch? And, not everyone who practices Witchcraft likes or wants to be called a Witch!
Witchcraft itself is a praxis: practice as distinguished from theory; the application and use of (practical magical) knowledge and skills; conventions, habits, and/or customs. It can—and often is—mistaken for a religion. However, Witchcraft can be practiced by anyone of any faith, or no faith at all.