Waterfall model is the most well-known model in software development also known as the traditional software development lifecycle.
 
The waterfall or the linear sequential model illustrates a sequenced systematic approach, which starts with analysis and progresses through each stage to testing and maintenance/completion.

Each stage has a set of defined milestones and results, and the progress to another stage does not occur until these predefined results are accomplished. A review is made at the end of each phase to determine whether the team can advance to the next. If the review gives a negative result, the team remains in the same stage until all required is completed successfully.

The waterfall model in software development is best suited to environments with stable product definition, for example, building a well-defined maintenance release of an existing product or porting an existing product to a new platform.

Problems with the Waterfall Model

• Real projects rarely follow the sequential flow that the model proposes.
• Although the linear model can accommodate iteration, it does so indirectly as a result; changes can cause confusion as the project proceeds.
• It is often difficult for the customer to state all requirements explicitly.
• A working version of the programs will not be available until late in the project time-span.
  A major problem, if un-detected until the working program is reviewed can be disastrous.

 Waterfall model forms the basis for other software development lifecycle variations.