UX Glossary · W
Waterfall
A sequential process where each phase completes before the next begins, with design finished before build.
Waterfall describes a development process running in fixed sequence: requirements, design, implementation, verification, maintenance. Each phase completes and is signed off before the next starts.
It is usually traced to a 1970 paper by Winston Royce, which is one of software’s better ironies: Royce presented the sequential model in order to criticise it, and argued for iteration and prototyping. The diagram was adopted and the argument was not.
Where it genuinely fits
Work where requirements are stable, verification is external and change is expensive: regulated medical devices, aerospace, safety-critical control systems, and anything with a certification gate. In those contexts the discipline of a fully specified design is a feature rather than an obstacle.
Automotive HMI work often sits closer to this end, because a supplier builds from a specification and a change late in the cycle is genuinely expensive.
Where it fails
Anywhere the requirements are actually assumptions. The cost of being wrong accumulates silently until verification, which is the last and most expensive place to discover it.
In practice
Most real projects are neither. A common shape is a specified frame with iterative work inside it: the architecture and the interfaces are fixed, and the details are tested and refined. Insisting a project is purely one or the other is usually a sign nobody has asked which decisions are actually reversible.