English dictionary for download [duplicate]
As far as I know, the standard is still Princeton's Wordnet, which has been converted into virtually every type of list, database or system imaginable, from Common Lisp Package to SQL database and everything inbetween.
Even if you are unfamiliar with every available technology, downloading virtually any of them will include downloading the wordnet database files which are recognizable by their db file extension. If you use MS Windows, try a program called WordWeb which is simply a Graphic User Interface for Wordnet; if you use Linux, try GoldenDict, a similar GUI for Wordnet.
From Linux, Unix and OSX, Wordnet can be used from the console via its Command Line Interface, usually simply called "wordnet" or "wn" in your software repositories.