As I’ve said many times, I consider abo-abo’s define-word one of my most useful packages. I use it several times a day. If you’re looking for a dictionary that ties in seamlessly with Emacs, you should take a look at define-word.
Some people may be put off by the fact that define-word leverages the Wordnik dictionary site to get its definitions. As I noted in my post on powerthesaurus, I’ve used define-word since it was announced (over 3 years ago) and have never had a problem with its depending on a third-party site. I guess that if I lost or didn’t have connectivity for some reason, I could just use the macOS system dictionary but I’ve never had to do that.
For any macOS Emacs users who are worried about such things, Xu Chunyang has written a package to query the macOS system dictionary from Emacs. I am, as I said, perfectly happy with define-word so I haven’t installed osx-dictionary. I could, I suppose, install it as a backup system in case I don’t have connectivity but, at least at this point, it doesn’t seem necessary.
But don’t let my preferences sway you. If you’re looking for an easy way to access the macOS system dictionary from Emacs, you should definitely take a look at Xu’s package.