Alistair Cockburn is one of the original signatories of the Agile Manifesto. He posted his comment to a thread on James’ blog in which he produces two definitions which I think nicely sum of the ideas of Agile. (Again, not surprising given the source.)

Agile methodology – a system of methods designed to allow the development team to match and track the business needs, especially in a context where business needs change frequently, important facts change, or where we are obliged to adapt to important uncontrolled factors.

Non-Agile methodology – one that optimizes toward a different priority, for example, seeking to achieve cost efficiency by anticipating, controlling, or eliminating variables so as to eliminate the need for changes and associated costs of changing.

By no means should these be considered the canonical definitions of such, as he prefaced them with taking my life in my hands, here goes with a tentative version based on the above line of reasoning, but I like them, and it’s my blog. 🙂