Agile software development is a conceptual framework for software engineering that promotes development iterations throughout the life-cycle of the project. – Wikipedia
Agile is an iterative and incremental (evolutionary) approach to software development which is performed in a highly collaborative manner by self-organizing teams with "just enough" ceremony that produces high quality software in a cost effective and timely manner which meets the changing needs of its stakeholders. - Scott W. Ambler in Agile Modeling
Agile development provides a set of practices simple enough to engage developers, managers, and customers yet sufficiently sound and disciplined to build quality software with predictability. - Dave Thomas (founding director of AgileAlliance.com)