David M. Schneider in the context of "Fictive kinship"

Fictive kinship (less often, fictional kinship) is a term used by anthropologists and ethnographers to describe forms of kinship or social ties that are based on neither consanguineal (blood ties) nor affinal ("by marriage") ties. It contrasts with true kinship ties.

To the extent that consanguineal and affinal kinship ties might be considered real or true kinship, the term fictive kinship has in the past been used to refer to those kinship ties that are fictional, in the sense of not-real. Invoking the concept as a cross-culturally valid anthropological category therefore rests on the presumption that the inverse category of "(true) kinship" built around consanguinity and affinity is similarly cross-culturally valid. Use of the term was common until the mid-to-late twentieth century, when anthropology effectively deconstructed and revised many of the concepts and categories around the study of kinship and social ties. In particular, anthropologists established that a consanguinity basis for kinship ties is not universal across cultures, and that—on the contrary—it may be a culturally specific symbol of kinship only in particular cultures (see the articles on kinship and David M. Schneider for more information on the history of kinship studies).