Paragon Software Group has launched the Oxford Advanced Learner’s Dictionary for Mac OS X. It offers explanations written using the 3000-word defining vocabulary.

The dictionary includes: 3,000 keyword entries designed to help you learn the most important words in English; over 57,000 synonyms and opposites; over 83,000 collocations that show how words are used together; information about British and American culture; 1,000 new words and meanings explain today’s vocabulary and usage; 1,300 words illustrated in groups help to build vocabulary for talking about topics. Illustrations can be enlarged and explored; over 95,000 extra example sentences and more.
You can listen to real British and American voices pronounce words and example sentences. You can access the high quality audio online, or download all 116,000 example sentences to listen offline.
 
The Oxford 3000™: This list of the 3,000 most important words to know in English was compiled by language experts and is corpus-based. The Oxford3000 words are marked with a key so that users can check they know the words, and can use the information in the entries to extend their vocabulary to learn phrasal verbs, idioms, synonyms, etc.
 
Oxford Advanced Learner’s Dictionary requires Mac OS X 10.7 or higher and is available for US$29.99 at https://itunes.apple.com/us/app/id553809551?mt=8 . It's also available for iOS devices at the Apple App Store.