What is the Agile Core? Practices and/or Values

At the last meeting there was a questions: why is waterfall so easy to describe, but it is so hard to describe Agile?

This is a problem because things that are easy to describe spread faster and broader than things that are hard to describe.

So is there a core to Agile that is easy to describe?

Are there some practices that are simply non-negotiable?

How would you describe Agile to someone who had never heard of it? How would you describe it to a skeptic?

If talking through these questions around the table sounds like fun, come on down and join us.