When is Software Development agile?

The following aspects characterise agile software development :

• Parts of a system are developed at different times. The system is always being enhanced by already accomplished parts. This is called incremental development.

• Defect analyses and measurements are used as learning opportunity by team members and for improvements of the organization.

• All involved parties / persons work close together and cooperate directly, e.g. by a high degree of teamwork and a continuous cooperation of the customer or product owner. 

Actually models for agile development exist since the 90s. They follow processes and, due to the mostly short iteration cycles (sprints), they heavily rely on measurements of progress, code quality, test coverage, etc. and a fast feedback of it to the developers.