The term interaction can be referred to as a for
m of communication, either one-on-one (telephone call), one-to-many (YouTube) or many-to-many (the Internet). Interactive experiences can be in any medium such as live performances and services, but primarily centres around technology. The discipline aims to define the behaviour of these complex systems which enable user interaction.

"Interactive design is concerned with a user, customer, audience or participant's experience flow through time... Interactivity is concerned with being part of the action of a system or performance..."
Nathan Shedroff

Related Links:
Nathan Shedroff- Information Interaction Design: A Unified Field Theory of Design
Bill Verplank Interview from Bill Morridge's "Designing Interactions"
Gillian Crampton Smith Interview from Bill Morridge's
"Designing Interactions"