I hear this term used quite frequently, but have yet to see it specified (and can't find it by searching). Single starting hands are pretty straight forward. Here, I'm using pokerstove syntax as an example:
Where things get a bit dicier is when it starts to take on a set-builder-like syntax, with ranges, and unions between ranges. (e.g.: 22+, A9s+, AKo for any pair, suited aces >= A9, and AKo).
As far as I see, people tend to use slight variations on the PF form, but the term "canonical form" seems to suggest that someone has at least started looking toward simplifying and/or standardizing it.
On one hand, Stove is ubiquitous enough that duplicating Prock's syntax isn't horrible, but I'd like to implement a standard if one exists.
Seems the answer is 'no' and that Pokerstove's syntax is ubiquitous enough that it can be used.
Feel free to join in fleshing out the syntax.