One of the most beautiful definitions about Interaction design is Mr. Bill Verplank´s model. According to Mr. Verplank, Interaction design involves answering three simple questions:
"1. How do you do? What sort of ways do you affect the world: poke it, manipulate it, sit on it? 2. How do you feel? What do you sense of the world and what are the sensory qualities that shape media? 3. How do you know? What are the ways that you learn and plan (or perhaps, how we want you to think)?"
I personally find the first question as the main column of this definition: Interaction implies touching, manipulating, poking things. The second and the third one put on the table the role of emotions and experiences, working in synergy but maybe playing a less important role.
This definition is even more powerful when narrated by him, drawing explicative sketches while defining the concept. You can see him in action at the promotional website of the Designing Interactions´s book.