gnuciDictionary is an offline nuget port of the GNU Collaborative International Dictionary of English. Defining a word is simple: Simply give a string, and you will be given a list of definitions for that string, or null if no definition can be found.

IEnumerable<gnuciDictionary.Word> definitions = gnuciDictionary.EnglishDictionary.Dictionary.Define("cat");

This will return an enumerable of Words, which contain information like what kind of word it is, what the definition is, and what the plural is.

cat: Any animal belonging to the natural family Felidae, and in particular to the various species of the genera Felis, Panthera, and Lynx.
Under The Hood

Word definitions are stored as compressed JSON files (“/data/gzip”). Each file is called something like dict_al.dat – the al here indicating that this file contains all words starting with those two letters. This allows gnuciDictionary to have a almost zero memory footprint as well as low disk storage footprint (~5MB).