Longman Dictionary of American English
http://www.pearsonlongman.com/ae/ldae/content.html
WEB52,000 words and phrases. Interactive CD-ROM with full contents of the dictionary. The Longman Defining Vocabulary, limited to the 2,000 most common English words, guarantees that definitions are easy to understand. 3,000 key words are highlighted so that students know exactly which words to learn first.
DA: 22 PA: 54 MOZ Rank: 89