Booth and Hess, 1974
We define friendship as a primary relationship between two individuals unrelated by kinship.
Booth, Alan and Elaine Hess 'Cross-Sex Friendship' Journal of Marriage and Family , Vol. 36, No. 1 Feb., 1974:38
A relationship involving voluntary or unconstrained interaction in which participants respond to one another personally.
Wright, P. H. 'Self-referent motivation and the intrinsic quality of friendship'. Journal of Social and Personal Relationships, 1, 115–130. 1984:119
Voluntary interdependence between two persons over time, that is intended to facilitate social–emotional goals of the participants, and may involve varying types and degrees of companionship,intimacy, affection, and mutual assistance.
Hays, R. B. "Friendship". In S. W. Duck (Ed.), Handbook of personal relationships: Theory, research, and interventions (pp. 391–408). New York: Wiley. 1988:395
A friend is defined specifically as someone with whom one has a relationship unprompted by anything other than the rewards this relationship provides.
Giddens, Anthony. Modernity and self-identity: Self and society in the late modern age. Stanford University Press, 1991:90