Longman : Dictionary of American English : The Key to Vocabulary Building.

This clear and comprehensive American English dictionary features: 109,000 words, meanings and phrases, including words from the Academic Word List; 59,000 example sentences, showing thousands of academic words and collocations; Clear definitions written using the Longman American Defining Vocabulary of just 2,000 common words; and words for content areas such as science and social studies. As a key to vocabulary building, it also contains: collocation boxes showing combinations of words that are often used together, such as strong interest, highly successful, and take a test; an integrated thesaurus explaining thousands of synonyms and antonyms; 3,000 etymologies explaining the origin of words (15,000 etymologies online); a learner's handbook on writing, collocations and grammar; and, workbook exercises for self-study and classroom use.