The essence of any interpersonal relationship is interaction. Two individuals may be said to have formed a relationship when on repeated occasions they are observed to interact. By interaction it is meant that they emit behavior in each other's presence, they create products for each other, or they communicate with each other. In every case we would identify as an instance of interaction there is at least the possibility that the actions of each person affect the other.

p. 10 - The social psychology of groups. 1959