Artha is a free cross-platform English thesaurus that works completely
off-line and is based on WordNet. Artha is a handy thesaurus that
focuses on high usability, without trading off simplicity and ease of
use. It has the following distinct features that increases its
usability: hot key lookup, regular expressions search, notifications,
suggestions, relative to sense mapping.

Homepage:
https://artha.sourceforge.net/
