An unpleasant experience that occurs when a person’s network of social relations is deficient in some important way, either quantitatively or qualitatively. This definition shares three important qualities. First, loneliness results from deficiencies in the person’s social relations. Second, loneliness is a subjective phenomenon that is not necessarily synonymous with objective isolation – people can be alone without being lonely). Third, loneliness is unpleasant and distressing.
Perlman, D., & Peplau, L. A. (1981). Toward a social psychology of loneliness. In Gilmour, R. & Duck, S. (Eds.). Personal Relationships Vol 3: Personal Relationships in Disorder, 31-56. London: Academic Press.