An Etymological Dictionary of Astronomy and Astrophysics

General: Mutual or reciprocal action or influence.
Physics: 1) What happens between a system and its surroundings when an observable change in one corresponds to or correlates with an observable change in the other.

2. The natural phenomenon of the transfer of energy between two particles. The
   four known kinds of interactions, in the order of increasing strength, are: → gravitational, → weak, → electromagnetic, and → strong.

See also: Interaction, from → inter-; + → action.