Linear, sequential software development process
The Waterfall model is a traditional software development methodology where each phase — requirements, design, development, testing, deployment — must be fully completed before the next begins. It contrasts with Agile's iterative approach and is rarely used in modern SaaS.
A company builds a 12-month product roadmap using waterfall. By launch, the market has shifted and half the features are irrelevant. This is why most startups choose Agile.