Wordsworth can be forgiven. If the word is not regarded as normative, obviously I would not push the criterion. But then equally obviously we would need a separate argument for caring about imagination. Thanks for your message. I'll take the qs.
Imagination is the ability to think flexibly and effectively about the possible.
That is the subject of the first chapter of the book. I'm not sure I can add much to it. The first part of this question is what I address in Chapter 2 of the book. Again, I can't think of much to add. And the second part of the question is what most of the rest of the book addresses.