In life, a relationship is a state of connection or association between two or more individuals. [1] In the world of the personal, of work, and of the world at large, relationships between people are a decisive force. [2] In the scientific perspective, relationships are often quantified by their interaction components.

Interactions
In sociological-psychology, a relationship is considered as a long interaction or function of a set of multitudes of microsecond interactions. An interaction between two individuals involves, at a minimum, individual A showing behavior X to individual B. Individual B may, in turn, respond with behavior Y. There may be a number of such repetitions of this sequence, involving behavior that is consistent or different, but an interaction is essentially limited in time. [3] A relationship, subsequently, involves a series of such interactions between individuals who know each other, such that each interaction is affected by preceding ones and usually by the expectation of future interactions. [4]

References
1. (a) Relationship (definition) – Dictionary.com
(b) Relationship (definition) – Merriam-Webster Collegiate Dictionary (2000).
2. Gilbert, Roberta, M. (1992). Extraordinary Relationships – a New Way of Thinking About Human Interactions, (pg. 3). New York: John Wiley & Sons.
3. Auhagen, Ann E. and Salish, Maria v. (1996). The Diversity of Human Relationships, (pg. 9). New York: Cambridge University Press.
4. (a) Bateson, G. (1979). Mind and Culture: a Necessary Unity. New York: Dutton.
(b) Rogers, L.E. and Millar, F.E. (1988). “Persuasion in Personal Relationships”, in S. Duck (Ed.), Handbook of Personal Relationships (pgs. 289-306). Chichester, England: Wiley.