In Agile development, where development is repeated in short periods, the key unit of the development cycle is called an iteration. Iterations, consisting of Design, Development, Testing, and Improvement are usually set for 1 to 4 weeks, and they are characterized by completing a full cycle of system development. After completing one cycle and releasing it, known as Iteration 1, the process is repeated with Iteration 2, Iteration 3, and so on.the.