Interaction Design is the definition, design, and study of technology and interactive systems upon users. It can also be the study of the actual relationship between an object or idea and a user. The goal of Interaction Design is to create a product or obtain a goal that makes the user's relationship with an object "easy" to use, learn, and manuver.
Take for example, a coffee mug. The perfect coffee mug is designed to hold the "perfect" amount of liquid. But it is also designed to pour into your mouth without spilling, not burn your hand, and even fit your hand in a comfortable manner. If all these things are found in a mug then its design was a good design and your use of this object is possibly a constant and enjoyable process.
The study and practice of Interaction design usually revolves around electronic technology, but it can expand to, "other products and services, and even organizations themselves."
Interaction Design is the acutal design, study, and implication of an object.
References:
http://en.wikipedia.org/wiki/Interaction_design
