I use the Sage.
From the site: TheSage's English Dictionary and Thesaurus
TheSage's English Dictionary and Thesaurus is a professional software
package that integrates a complete dictionary and a multifaceted
thesaurus of the English language into a single and powerful language
reference system.
Features
Some of the most interesting features of TheSage are:
Features Status Notes
Comprehensive dictionary Growing Over 145,000 references with multiple
detailed definitions (over 200,000)
Complete thesaurus Growing Nearly 1,200,000 relationships between
definitions (synonyms, antonyms, hypernyms, hyponyms, meronyms,
holonyms,...)
Information integration Implemented Each definition is directly linked
to its own thesaurus.
Example sentences Growing A large collection that already includes
approximately 35,000 examples of usage.
Price Free! Absolutely 100% free of charge.
Cross-referencing Implemented Any and all displayed words are
clickable, triggering a new lookup.
Wildcard search Improved Match single/multiple characters as well as
filter by single/groups of vowels and consonants.
Anagram search Implemented Only valid English words are returned.
Clipboard support Improved Copy search results, examples, thesauri,
single/multiple definitions, even the entire lookup.
Tabbed interface Implemented Previous lookups and searches remain
readily available.
Structured display Implemented Clean, flexible, fast, and easy to use
presentation of information.
Custom appearance Improved Choose your own font and color for each
aspect of the display.
Multi-session History Implemented Quickly available via dropdown or the
'History' tab.
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