Online references for vocabulary

Many well-established dictionaries have online versions now, not all of them free:

• The online Merriam Webster dictionary (and thesaurus).
• The online Oxford English Dictionary; this requires a subscription.
• The online Cambridge Dictionary.

A user-contributed dictionary—along the lines of Wikipedia—is:

• Wiktionary, the free dictionary.

A dictionary that attempts to keep up with current—but possibly only transient—coinages, is

• Urban Dictionary.