In
Software Engineering,
The Waterfall a
software lifecycle model where you progress linearily along the
project phases in a manner similar to this:
Requirements
Architecture
Design
Construction
Testing
Deployment
You're theoretically supposed to enter the next phase only after you finish the previous one, as well as thoroughly document the deliverables of each one.
This makes for easy WBSes and Project Management, but doesn't usually work exactly well in real life, as requirements change mid-project, and schedule compression requires to begin testing or deploying an unfinished product.
This is somewhat resolved in the so called Salmon Waterfall where you can carefully return to the previous phase, and/or the Sashimi Waterfall, where the phases are permitted to overlap. Some say that such have been the original intentions of its proponents (Structured Design zealots, such as Wirth and Yourdon, I'd presume) in the first place.
The alternatives are Extreme Programming, spyral, prototyping and others.
Written from memory, but an extensive analysis of this is found in Wicked Problems, Righteous Solutions by DeGrace and Stahl.