Have reptiles been formally defined, and if so, what are they?
I've heard them referred to as an unnatural group (diapsids excluding dinosaurs), and - of course - as synonymous with "diapsid", but the thing that inspired me to ask was the fact that non-mammal synapsids are referred to as "mammal-like reptiles". So if you include all descendants of the common ancestor of those animals commonly referred to as reptiles, we should be they (so Reptilia = Amniota). Aesthetically, i like this (because the presence of the term "mammal-like reptile", and also: in my little brain "reptile" conjures up the concept "tetrapod adapted to dryness" more strongly than it does "diapsid" (and even more strongly than "outdated paraphyletic group"), but i thought i'd better ask to see how things really are.
So has it been formally defined, or is it one of those common words whose appropriateness everybody has a different idea about?