Sources I've inspected include:
- OMG MDA Guide 2.0, 2014;
- MDA distilled; principles of MDA, Mellor et al. 2004;
- MDA with Executable UML, Raistrick et al, 2007;
- Model Driven Engineeirng and Ontology Development, Gasevic et al, 2009
So currently my thinking is to pull a concise summary of such key aspects together, as as close an approximation to a definition as we can find (and others appear to have done).