What Interaction Is
Interaction is the exchange between a user and a system — every input the user makes and every response the system returns is part of that conversation.
Every interface is a conversation. The user acts. The system responds. What happens in that exchange — and whether the user understands it — is what interaction design is about.
What it is
Interaction design is the discipline of defining how a system responds to human input. It covers what happens when a user clicks, taps, types, scrolls, or speaks — and what the system communicates back. Unlike visual design, which concerns how an interface looks, interaction design concerns how it behaves. The two are inseparable in practice, but the distinction matters: an interface can look correct and behave incorrectly.
The input–process–output model
Every interaction follows the same structure. The user provides input — a gesture, a keystroke, a voice command. The system processes it — validates, computes, retrieves. The system returns output — a changed state, a message, a new screen. Good interaction design makes each of these steps legible: the user knows what input is expected, what the system is doing, and what the output means. Failures in any stage produce confusion.
What interaction design is not
Interaction design is not animation. Motion can support interaction — conveying state changes, guiding attention — but it is not the substance of the discipline. It is also not UX research, though research informs it. At its core, interaction design is about defining the behaviour of controls, states, and system responses so that users can accomplish their goals predictably.
Why behaviour precedes aesthetics
A button that looks perfect but does not communicate its state, a form that takes input without confirming submission, a modal that appears without any cue as to how to dismiss it — all of these are interaction failures that no visual treatment can fix. Behaviour must be designed before it can be styled. The same principle applies here as in form before style: solve the structural problem first.
The takeaway
Start with the exchange. Before deciding how a control looks, decide what it does, what state it communicates, and how the system responds when it is used. The visual design of the control follows from those decisions — not the other way around.