The waterfall model is a sequential software development model (a process for the creation of software) in which development is seen as flowing steadily downwards (like a waterfall) through the phases of requirements analysis, design, implementation, testing (validation), integration, and maintenance. The origin of the term “waterfall” is often cited to be an article published in 1970 by Winston W. Royce (1929–1995),[1] although Royce did not use the term “waterfall” in this article. Ironically, Royce was presenting this model as an example of a flawed, non-working model (Royce 1970).

Together with the paper on MANAGING THE DEVELOPMENT OF LARGE SOFTWARE SYSTEMS by Winston W. Royce this is an interesting read.