[dxwg] Improve and consider moving definitions

nicholascar has just created a new issue for https://github.com/w3c/dxwg:

== Improve and consider moving definitions ==
(Antoine:) I'm pasting Nick's definitions here. They need to be polished (as Nick has written himself) and I think we may want to move the definition of profile into the introduction. In any case I'm keeping them entirely for now as it's good to have a reference set of terms we've been interested in defining at some point and I think we should welcome input from the WG, on whether we should add other terms here. For example, 'schema', as referred at https://github.com/w3c/dxwg/issues/242#issuecomment-408916364

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