Rapid developement iterative development model

In iterative development, the same waterfall process is used to deliver smaller chunks of functionality in a step-by-step manner. This reduces the management and overheads in delivering software and reduces the risk inherent in the project.
One of the major reasons cited for software project failure (and a common
sources of defects) is poor quality requirements. That is a failure to correctly
specify what to build.

By delivering small chunks and validating them, the project can self correct
and hopefully converge on the desired outcome. This contrasts with the long lag times in the waterfall model.

A variation on this theme is “Rapid Applications Development” or RAD.
The phases are similar to waterfall but the 'chunks' are smaller. The emphasis in this model is on fast iterations through the cycle. Prototypes are designed, developed and evaluated with users, involving them in the process and correcting the design. The model is particularly suited to projects in rapidly changing environments where the team needs to adapt to different situations.