Interaction Design (IxD) is the design of the interaction between users and products. It focuses on the behaviour of the product and how it responds to user inputs (clicks, gestures, voice). It encompasses the structure, feedback, and flow of the interface.
Good interaction design makes a product feel responsive and alive. It prevents errors by restricting invalid actions (constraints) and communicating system status clearly (visibility). It is the difference between a static page and a functional tool.
Action implies reaction. Every user input must have an immediate, perceptible acknowledgment (latency < 100ms). We design for "affordance"—the interface should intuitively suggest how it is to be used without requiring instructions.