thesaurus

Look up words in Pages on Mac - Apple Support should work.

There is no built-in Thesaurus in Pages, but you can add a Thesaurus to the Apple Dictionary Preferences:

And these will be left most in the Apple Dictionary toolbar when you launch it:

And if you control/right-click on a word and choose Lookup ___, the preceding dictionaries results will be shown in the same order. This ordering is also true when you hover over a word and use control+command+d, or a three-finger tap (if configured in Track Pad preferences).

I keep the Apple Dictionary application centered in my Dock.