A gem that Apple has added with iOS 12 is the perfect companion to the dictionary, which is already available with the "Look Up" tool. I say "gemstone" because it's on all other new iOS 12 features, but it should not, because a thesaurus is like the phloem for the xylem of a dictionary, making it hard to believe that Apple does not have that synonym-finder included from the beginning.
The only problem with Apple's new integrated thesaurus is that it's not enabled by default. At least it was not in the first developer beta for iOS 12. It seems that the second developer beta, as well as the first public beta release, is enabled by default, so it may stay that way when iOS 1
So, if you write an in-depth scholarly article about how OXO biodegradable plastics are not really as good as they sound and want to swap the "phase" for another word, consider using the "lookup" tool See only results from the New Oxford American Dictionary or Apple Dictionary by default. If this is the case, you must unlock the thesaurus. If you see the Oxford American Writers Thesaurus then you can stop here, because it is already activated for you.
If you do not see it, it's pretty easy to add. After long pressing on a word or double-tapping, depending on whether you are reading or editing text, select "Lookup" from the black context menu. On the overlaid page, swipe all the way down and tap Manage Dictionaries. (You can also access these settings through Settings -> General -> Dictionary.) Make sure that Oxford American Writers Thesaurus was reviewed in the Settings Dictionary ] Well, if you're long Tap a word or tap it twice, depending on whether you are reading or editing text, tap Search in the menu that appears, and you will see the new thesaurus under the Dictionary section above. Tap to expand the full list of available synonyms. If you do not see the thesaurus, it is probably because there were no results.
This is a nice addition to the lookup tool in iOS and is certainly better if you have money for the Oxford Author's Oxford Thesaurus book or one of Oxford's many thesaurus apps for iOS. There are many free thesaurus apps available from other publishers, but integrating this into iOS is the most convenient option.