UX Glossary · A
API
Also: Application Programming Interface
The defined way one piece of software talks to another, and a real constraint on what an interface can show.
An API is the contract between two pieces of software: what one can ask for, in what format, and what it gets back. Designers do not write APIs, but APIs decide what a design can actually do.
Why it matters to a designer
Three constraints show up constantly.
What data exists. A dashboard can only show what the API returns. Designing a filter on a field the API does not expose produces a design that cannot be built.
How data is shaped. If an endpoint returns customers and a separate one returns their orders, a screen showing both means two calls, which means two loading states and two possible failures.
How fast and how reliably. An endpoint taking 2 seconds needs a designed waiting state. One that fails sometimes needs a designed error and a way to retry.
In practice
This is why specifying the data layer alongside the interface pays off. A chart is a claim about what the data can do, and making that claim without checking it is how a design gets approved and then turns out to be unbuildable. Asking “where does this number come from” early is a design question, not an engineering one.