The waterfall model, also known as the linear sequential model, is a software development approach where each phase must be completed fully before the next begins. It is suitable for projects with well-known and stable requirements, offering advantages such as ease of implementation and early error detection, but has significant drawbacks including unsuitability for larger projects and difficulty in accommodating changing requirements. The model was first formally described in a 1970 article by Winston W. Royce, who presented it as a flawed model yet it remains widely used in software projects.