Software Architect's Handbook

Adaptive rather than predictive

Agile methodologies are adaptive rather than predictive. While methodologies such as Waterfall focus on a large amount of upfront planning to detail and predict outcomes in the future, Agile methodologies focus on adapting to changes as they occur.

Although it becomes difficult to know what the state of the software will be in the future, and increasingly so the further into the future you want to examine, Agile methodologies allow you to make changes after the initial planning takes place. Feedback from users and testers can be taken into consideration immediately, and incorporated into the work of subsequent iterations.