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The various activities which are undertaken when developing software are commonly modeled as a software development lifecycle (SDLC). The software development lifecycle begins with the identification of a requirement for software and ends with the formal verification (Software Testing) of the developed software against that requirement.
The software development lifecycle does not exist by itself; it is in fact part of an overall product lifecycle. Within the product lifecycle, software will undergo maintenance to correct errors and to comply with changes to requirements. The simplest overall form is where the product is just software, but it can become much more complicated, with multiple software developments each forming part of an overall system to comprise a product. | 
