One gem that Apple added with iOS 12 is the perfect companion for the dictionary that's already available with the "Look Up" tool. I say gem because it's taken a backseat to all of the other new iOS 12 features, but it shouldn't, because a thesaurus is like the phloem to a dictionary's xylem, making it hard to believe that Apple didn't include this synonym finder from the start.
The only problem with Apple's new built-in thesaurus is that it's not enabled by default. At least, it wasn't in the first developer beta for iOS 12. It appears that the second developer beta, as well as the first public beta, has enabled it by default, so that might stick when iOS 12 is released to everyone in the fall.
So if you're writing an in-depth scientific article about how OXO-biodegradable plastics aren't really as good as they sound and want to swap "phase" out for another word, when you use the "Look Up" tool, you might only see results from the New Oxford American Dictionary or Apple Dictionary by default. If that's the case, you need to unlock the thesaurus. If you see the Oxford American Writer's Thesaurus, then you can stop right here, because it's already enabled for you.
If you don't see it, it's pretty easy to add. After long-pressing on a word or double-tapping it, depending on if you're just reading or editing text, select "Lookup" from the black context menu. On the overlayed page that appears, swipe all the way down to the bottom and tap "Manage Dictionaries." (You can also get to these preferences by going to Settings –> General –> Dictionary instead.) Once in the Dictionary settings, make sure Oxford American Writer's Thesaurus is checked.
Now, whenever you long-press on a word or double-tap it, depending on if you're just reading or editing text, respectively, tap on "Lookup" from the menu that appears, and you'll see the new thesaurus under the Dictionary section up top. Tap on that to expand the full list of synonyms available. If you don't see the thesaurus listed, it's probably because there were no results.
This is a nice addition to the Lookup tool in iOS and sure beats paying money for the physical Oxford American Writer's Thesaurus book or one of Oxford's many thesaurus apps for iOS. There are lots of free thesaurus apps available from other publishers, but having this built into iOS is the most convenient option.