What is the Agile Core? Practices and/or Values

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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.

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What is the Agile Core? Practices and/or Values