UX Glossary · I
Interaction design
Also: IxD
Designing how a product behaves: what it does in response to input, over time, in every state.
Interaction design is concerned with behaviour rather than appearance: what happens when someone acts, what the system does while it is working, what it does when it fails, and how one state becomes another.
Bill Moggridge and Bill Verplank coined the term in the 1980s. Alan Cooper’s later work, particularly About Face, did much to establish it as a distinct practice from visual design.
What it actually covers
The parts of a product that no static screen shows. Loading, empty, error and partial states. What happens when a value is invalid, and when the user finds out. Whether an action is reversible and how. What happens with 0, 1 and 10,000 items.
A UI kit will give you a button. Interaction design decides what the button does when it is pressed twice, when the network is slow, and when the operation cannot be undone.
Why it is the expensive part to get wrong
Visual problems are visible and get fixed. Behavioural problems surface as support tickets, abandoned tasks and workarounds, and they are frequently attributed to users rather than to the design.
In practice
The useful discipline is enumerating states before designing screens. Most interfaces are designed for the happy path and then patched for everything else, which is why the everything else feels like an afterthought.