This website contains age-restricted materials including nudity and explicit depictions of sexual activity.
By entering, you affirm that you are at least 18 years of age or the age of majority in the jurisdiction you are accessing the website from and you consent to viewing sexually explicit content.
It’s a term in the artist community to refer to characters that are standalone, as in they aren’t from a particular work of fiction, which are in turn stereotypically (but not always) used for purposes of roleplaying.