The UIUC Library has a large collection of print dictionaries. To find a monolingual English dictionaries, search the Library's Primo Catalog using the subject heading English Language Dictionaries and selecting "subject" or searching using the keywords English Dictionary.
Merriam-Webster's Collegiate Dictionary, Eleventh Edition
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Etymological and historical dictionaries
Web Sites
Academic Word Lists - compiled by a scholar in New Zealand, these lists of commonly used academic words are organized by usage frequency.
Academic English Cafe - there are vocabulary quizzes as well as spelling, writing, and grammar quizzes. The writing prompts may also be useful for practicing persuasive and descriptive writing skills.
Word A Day - a new, unusual English word is posted each day with a definition, audio pronunciation, and example usage. Great for native speakers and advanced language learners alike!
200 Vocabulary Exercises - visual exercises in vocabulary from the University of Victoria English Language Centre.
WordSift is a new online tool for visualizing new vocabulary. A review article in Education Week describes how the tool works.