I think it's worth linking to the original Agile Manifesto[1], because that's pretty much all the consensus you're ever going to get on what's "agile" and "what's not".

Lewis is right that most of these principles were described before the manifesto, but I can vouch for the near-impossibility in many contexts of convincing anyone who wasn't a coder (and a lot of coders too) why these might be sensible defaults.

For every person burned by a subsequent maladaptive formalization of these principles, there was someone horribly scarred before the agile manifesto by being forced to go through a doomed waterfall process.