A development method in which successive versions are produced to respond to discoveries surfaced by the previous versions. It is applied when requirements are uncertain and/or when technology experimentation is required. The alternative method is linear development. (Forsberg, Mooz, Cotterman 2005, p 423)
Forsberg, K., H. Mooz, H. Cotterman. 2005. Visualizing Project Management, 3rd ed. New York, NY, USA: J. Wiley & Sons.
There is a substantial amount of discussion on evolutionary development in the section on Life Cycle Models.