Waterfall
In plain terms
A sequential, phase-by-phase project methodology.
Going deeper
Waterfall completes each phase, requirements, design, build, test, fully before the next, with little backtracking. It's linear and plan-driven.
Under the hood
Predictable for stable requirements but inflexible to change; largely replaced by Agile for software.
Examples
Traditional projects, fixed-scope contracts, regulated industries, pre-Agile.
Think of it like...
Like building a house strictly room by room with no going back once a phase is done.