What is the definition of "canonical"?

I just received a referee report criticizing that I would too often use the word "canonical". I have a certain understanding of what "canonical" should stand for, but the report shows me that other people might think differently. So I am asking:

1. Is there a definition of "canonical"?

2. What are examples where the use of "canonical" is undoubtedly correct?

3. What are examples where the use of "canonical" is undoubtedly incorrect?

VERY LATE EDIT: I just came across this wonderful passage written by R.Lipschitz, In a correspondence with André Weil (Oeuvres, vol. 2, page 558):

I can assure you, at any rate, that [...] my results are invariant, probably canonical, perhaps even functorial.