Interaction Design (IxD) defines the structure and behavior of interactive systems. Interaction Designers strive to create meaningful relationships between people and the products and services that they use, from computers to mobile devices to appliances and beyond. (from IxDA)
It includes the creation of behavior and communication tools and processes that facilitate communication between humans and objects, services, and physical or virtual environments, and that allow manipulation of those objects, services, and environments. It is important to study the flow of information and the discourse between the human and the interface including feedback and stimulus-response over a period of time.
Lifecycle: Interaction design