From Premier Assistive Technology:
Ultimate Talking Dictionary (UTD) is a comprehensive, unabridged dictionary. However, this is not a regular dictionary. UTD will read the definition of a word to you and it will use the word in a sentence so you get a true understanding of the word. The total dictionary that comes with UTD contains more than 250,000 words, including slang, jargon, and historic figures such as George Washington. The application also has a fully integrated thesaurus for referencing synonyms and antonyms of all words. Just highlight the word you want to search for in any text, press a single hot key and UTD looks it up for you, reads you the definition, and presents you with a list of synonyms.