Executable UML is at the next higher layer of abstraction, abstracting away both specific programming languages and decisions about the organization … - Stephen J. Mellor

I was astonished to be invited to what became the meeting that originated the Agile Manifesto because my work had always been based around building models... The other signatories were kind enough, back in 2001, to write the manifesto using the word “software” (which can include executable models), not “code” (which is more specific.) As such I felt able, in good conscience, to become a signatory to the Manifesto while continuing to promote executable modeling. Ten years on we have a standard action language for agile modeling.

What's the point of having metamodels, and why should you care? Because models must be stated in a way that yields a common understanding among all involved parties, we need a way to specify exactly what a model means. Metamodels allow you to do just that: They specify the concepts of the language you're using to specify a model.

In the bad old days before MDA, (conceptual) models served only to facilitate communication between customers and developers and act as blueprints for construction. Nowadays, MDA establishes the infrastructure for defining and executing transformations between models of various kinds.