The system “Look Up” works like in any other app, and provides access to the system dictionaries.
There are also many actions that can integrate with third party dictionary/thesaurus apps, like the aforementioned Terminology, but also Lookup and others.
Terminology also has a share extension, so you can select a word, select “Share” in the popup menu, and get a lookup. Some other third party dictionary apps also offer share extensions that work similarly.
Also, many web-based dictionary resources can be integrated, there are many examples in the directory, like Dictionary.com.
If you are looking for something built-in, that’s not something Drafts does. It does not ship with thesaurus/dictionaries, so you would have to integrate with some third party apps or websites. If you have a favorite resource, like a thesaurus website, I’m sure we could help you construct an action to do a lookup on that site.