Developer Chris Wayne has created a Unity scope that allows you to search Thesaurus using the same API of dictionary and lists the synonyms and antonyms of a word. The scope integrates well with Unity, and doesn't comes with a separate lens, which means you will be able to search words right from Unity home.
To install it:
sudo add-apt-repository ppa:scopes-packagers/ppa
sudo apt-get update
sudo apt-get install unity-scope-thesaurus