White Witch is a title occasionally given to cunning folk. These were workers of folk magic who charged money for removing the supposed effects of witchcraft. A white witch was not a subtype of witch, but a completely different class of person. While there is no evidence for the existence of witches, white witches were a matter of record.

You must know that some two or three years past there came to these parts one who called himself Doctor Doboobie, although it may be he never wrote even Magister Artium, save in right of his hungry belly. Or it may be, that if he had any degrees, they were of the devil’s giving; for he was what the vulgar call a white witch, a cunning man, and such like. - Sir Walter Scott, Kenilworth (1821)

The antonym black witch is an entirely modern creation as it was not previously needed. Witches were, without exception, malefic and appear to have been imaginary scapegoats on which diseases and bad luck could be blamed.

The term or a similar one, good witch, was in use from the 16th century but it was fairly uncommon in ordinary use except in Devon.

The recent rise of Wicca and other neopagan faiths has caused the term 'white witch' to fall into disuse, as neopagan revisionism attempts to classify all witchcraft as benign or at least morally neutral.