Online dictionary includes definitions from American Heritage, Webster's, Rogets Thesaurus and specialty definitions for things like abbreviations and legal terms. Links to many foreign language dictionaries.
Etymologies are not definitions; they're explanations of what our words meant and how they sounded 600 or 2,000 years ago. This has been compiled from numerous etymological dictionaries including Weekley's "An Etymological Dictionary of Modern English," Klein's "A Comprehensive Etymological Dictionary of the English Language," and "Oxford English Dictionary" (second edition).
A multi-language free-content online encyclopedia project. Since many people have created the content of the Wikipedia, it has a wide range of information, but the articles are often not edited, it is not always reliable and some instructors may not allow its use for academic work. Cross reference Wikipedia information with other reliable sources and check the entry footnotes.
The online version is the free version with short articles from the encyclopedia as well as Merriam-Webster's Collegiate Dictionary, Thesaurus, and the Britannica Book of the Year. For full length articles use the print edition.