At its essence, social interaction is defined as the manner in which people talk to and interact with one another. Social interaction is dependent upon the societal structure in which the communications occur.
Examples of societal structures in which social interaction occurs include everything from family units to churches to governmental agencies to educational institutions. In addition, a societal structure in which social interaction occurs need not be something that exists for an extended period of time. As noted, a social structure can include a long-standing organization like a church, but it can also include a group of friends out for a night on the town.
Social interaction also requires a mutual engagement. One individual spying or stalking another does not count as social interaction because the contact is not wanted by at least one party. This is also true of those people who commit crimes against other people. On the other hand, social interaction does not require both parties to be in the same room as one another, either. Writing a letter to a friend, two parties willingly speaking on the phone or two parties playing an Internet game all count as social interactions.