In agile software development, iteration refers to the specific time period set aside for the development of a product or a piece of software. Iterations vary from project to project but tend to be short. Most iterations last for just one or two weeks, although some can go as long as four weeks.
Agile principles assume that any one project will be made up of a series of iterations, agreed in advance by the development team and that projects work on an iteration-by-iteration basis.
Iterations give teams an idea, albeit a basic one, of how long a project has left to run.