Interaction is everywhere in the universe, ranging from an atom to a cell, from plants to animals, from human society to computer systems. It usually involves at least two entities communicating with each other.
Therefore, it must talk about communication when discussing interaction. To interact with each other smoothly, entities must be coordinated in terms of their activities.
We cannot avoid concurrency when talking about interaction, since interaction must take place between multiple concurrent existing entities.Those interactive entities can be modeled as Actors.
