Interactive Design - Definition

Interactive Design is classified as the relationship between the user and the system that they are interacting with. This interaction can be one to one such as the case of email, one to many apps like myspace, facebook or blogs and even many to many software.
This type of design focuses on the behavior of software and how users interact and make connections with the application.

IxDA.org says that "The success of products in the marketplace depends on the design of high-quality, engaging interactive experiences. "
IxDA also lists a number of hallmarks of good interactive design, these include:

• effectively communicates a system's interactivity and functionality
• defines behaviors that communicate a system's responses to user interactions
• reveals both simple and complex workflows
• informs users about system state changes
• prevents user error