I may be a spellcaster, but I'm not a witch anymore.
— Sylvia regarding her witch status
A spellcaster is term used to describe a person who performs magic and witchcraft, but does not identify as a witch. Thus far, the only person in the series who refers to themselves in this way is Sylvia Weald, who disowned the witches due to the Harmans (her paternal family) not allowing her to claim the name of Hearth-Woman and take a seat on the Night World Council, as Sylvia's Harman heritage came from her father, not her mother.
It can be assumed that any witch who rejects his or her heritage, but still practices witchcraft, may refer to themselves as spellcasters. Presumably, spellcasters, in addition to not calling themselves 'witches' and disassociating with them, also reject witch ideals, values and culture, in particular the reverence for all life (Sylvia allied herself with the lamia patriarch Hunter Redfern and initially tried to help him in his plan to overthrow the human race and destroy the witches, also abducting and torturing the Maiden of the Witches, Aradia, in the process, though she later had a change of heart). Because the majority of the witches seceded to Circle Daybreak in opposition to the Night World, it's possible that the witches who chose to remain loyal to the Night World (most of whom were from Circle Midnight) now see themselves as 'spellcasters' rather than true witches. Despite this however, spellcasters are technically still members of the witch species, no matter how much they may deny it.