Are macOS dictionary definitions accessible via the Web?

Dictionary sources can be both 1st party (provided by Apple) or 3rd party - user created.

Apple has a Dictionary Programing Guide available that describes how to create your own dictionary and even has a tutorial for creating a Japanese language dictionary.

You can add dictionaries via the Preferences (⌘ Command,.

Are the definitions available online?

Yes, but this is too broad for this venue. You will have to search for the specific dictionary you're looking for. Just be aware, it's up to the 3rd party if they want to provide one.

If you go to preferences for Dictionary you can see the source of the Dictionary and choose which source you want to use. I use the New Oxford American Dictionary.