Artha is an offline thesaurus - essentially a system-wide GUI front end for the huge Princeton WordNet lexical database (V3.0). You can more or less forget those useless, crippled dictionaries that come with ebook readers - this is what you need for US/UK English! Artha can provide:

Senses
Synonyms
Antonyms
Derivatives
Pertainyms
Attributes
Similar Terms
Domain Terms
Causes
Entails
Hypernyms
Hyponyms
Meronyms
Holonyms
Relative to Sense Mapping

Artha is desgined to be run primarily from the system tray. Highlight a word, press the designated hotkey combo (default is CTRL ALT W, in that order - and all together) and Artha goes to work. You can suppress the main window by clicking the Notify button in the main window. This will mean you get a shortened description in a balloon tip popup. Using the hotkey combo with no word highlighted calls up the main window. Artha accepts both US & UK spellings so you can use whichever spelling is natural to you and accepts wildcard characters - in case you need to cheat on the crossword.

Artha does not work with Kindle Cloud Reader or CoolReader, but works with a myriad of other pieces of software and now provides spelling suggestions for mis-spelt words.

Due to GTK limitations, we are stuck with V1.0.2 of Artha for now (current version = 1.0.3).