Iterative Model
There is a certain amount of backtracking in the actual evolution of software of any complexity, particularly if the software is constantly being updated.
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The iterative design model is based on the idea that design should be done in repeated cycles where, in each cycle, the design is elaborated, refined, and tested, and the results of testing at each cycle feed into the design focus of the next cycle.
This is identical in spirit to the notion of developing a software product through a series of continually-refined prototypes, and the idea of developing generations of a software product through an iterative development cycle (such as the Spiral model of development).
Prototyping is the development of incomplete representations of a target system for testing purposes and as a way of understanding the difficulties of development and the scale of the problem.
Prototyping is an essential element of an iterative design approach, where designs are created, evaluated, and refined until the desired performance or usability is achieved. Prototypes can range from extremely simple sketches (low-fidelity prototypes) to full systems that contain nearly all the functionality of the final system (high-fidelity prototypes).