Interaction is one or more instances of interacting relationship within a specific context.
Interactions contain identifiable input(s), action(s) and output(s), often including measurable changes to physical resources. However, agents rarely try to identify all inputs and outputs for interactions. (See also process).
Contexts for conceptual interactions vary greatly. By contrast, specific physical contexts tend to be extremely limited in spatial and/or temporal scale. However, that depends greatly on the scope and level of detail of potentially inter-acting elements and forms which agents attend to.
For example, we may confidently state that a cat ate a bowl of food which it came into contact with for a few minutes. The cat and the food are interacting elements within a relatively clear and compact scope from a human perspective, although that scope would be ineffably immense to a dust mite. Likewise, scientists may determine that a specific black hole is consuming a specific star. The black hole and the star are interacting elements, but their interaction occurs within an immensely larger scope than the cat and the food, because these interacting elements contain intense forces which act continuously and predictably over great distances. Such large-scale intensity and predictability is not the case with most interacting elements at typical scales of human experience.
Most physical interactions are extremely localized because of the attenuation of interacting forces, which insulates the active elements from most other potentially interactive elements. However, our predictions or simulations of most (ungoverned) interactions over long periods of time (a larger context) rapidly break down via the exponentially expanding complexity of interaction over time, as described in chaos theory. (Also, localization does not imply complete separation or isolation.)