English as a Second Language (ESL): Dictionaries

The Free Dictionary offers financial, legal and medical terminology dictionaries, acronyms, articles and words of the day, idioms, and spelling quizes.

This dictionary website is free to use, and includes audio clips of every word in the dictionary. The speakers have North American accents. Offerings include a dictionary, thesaurus, and encyclopedia, medical vocabulary, "word of the day", games, videos and slang options.

Vocabulary.com has a streamlined interface without advertisements. It offers narrative, in addition to academic, descriptions for the most frequently looked up words.

For example:
Empirical: If knowledge is empirical, it's based on observation rather than theory. To do an empirical study of donut shops, you'll need to visit every one you can find.
-Versus-
Empirical: derived from experiment and observation rather than theory.

There are 100 million sentence examples of the word in use from news, medicine/science, technology and other disciplines. Audio files of defined terms are available in computerized MP3 format.

While helpful, this site also uses a lot of vernacular (jargon) which may not be known to non-native speakers, in its definitions. Additionally, the sentence examples are not cited, making it difficult to go back to the source to read further.