I have converted two English-English dictionary to user dictionaries for use with the Pleco, the best iOS/Android Chinese-English dictionary available. Of course these aren't officially supported by Pleco, which has no English-English dictionaries at present, so any bugs or problems they cause are not the fault of the creators of Pleco. Why an English-English dictionary in Pleco? I can think of a few reasons why one might be useful:

• For a native Chinese speaker, it's just as handy as some of the great Chinese-Chinese dictionaries Pleco provides.
• When trying to translate a word from English to Chinese, sometimes having synonyms, and more/less specific words might be helpful.
• English speaking users can look up obscure words returned from a search in Chinese without leaving Pleco.
  
• Pleco is a great dictionary engine, and using any other English-English dictionary is painful after getting used to Pleco.

There hasn't been much demand for a commercial English-English dictionary in Pleco, as most Pleco users are native English speakers, so at the moment these free dictionaries fill the gap, They are quite different, and complement each other nicely.

If you're not familiar with Pleco's file manager and/or dictionary installation process, take a look at my How To Guide here that explains how to install these dictionaries.

A once authoritative dictionary, with many usage examples and quotations, now in the public domain and obtained from Project Gutenberg. It claims to have around 200,000 entries, although after merging homographs together I have ended up with 102,482 entries. I did a bit of work with regular expressions to clean up the dictionary formatting as it was converted. I have tried to add links to other words as appropriate (e.g. when "See also", "Same as", etc. link to another entry I have added a hyperlink). The original Webster's dictionary is full of abbreviations, and at times reading an etymology can be very difficult as it is simply a list of abbreviations. I used an abbreviation list from Wiktionary as the basis for some code that expanded these abbreviations, giving a much more readable result. For example "NL., fr. L. spira" in the original Webster's will read "New Latin, from Latin spira" in the converted version.

• Download PlecoWebsters20131118.pqb.zip (36 MB)

WordNet is the product of a very interesting project at Princeton, and as a dictionary it fills in some of the more modern terms that are missing from Webster's. The definitions are very terse, but the real advantage of WordNet lies in the links that it provides between words (Synonyms, Antonyms, Hyponyms, Holonyms, etc.). After merging entries together, The Pleco conversion has ended up with a dictionary 147306 entries. All of WordNet's relationships between words are provided in the Pleco conversion.

• Download PlecoWordNet20131118.pqb.zip (48 MB)

As with everything on this site, I am happy for it to be used non-commercially in any way. In addition, please respect the licenses of the source materials, that are available on their respective websites and that are distributed along with the dictionaries.