The "spiral model" is an iterative model for software development, described by BarryBoehm.  Each iteration basically follows the WaterfallModel, but subsequent iterations build upon previous work.

It is called a "spiral" because the common illustrative diagram shows a long string of waterfalls wrapping around a center, emphasizing the aspect of continuous growth and revisiting of previous work.  See http://www-db.stanford.edu/~burback/watersluice/node53.html for an example.

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See also ExtremeProgrammingIsJustTheSpiralModel
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CategoryApplicationDevelopment