Use the Big Huge Thesaurus API to get synonyms for words, as well as antonyms, related terms, and similar terms. This is a free, English language thesaurus with over 145,000 words. No API key required if you make fewer than 10,000 calls per day. Version 1 has simple output, and version 2 has enhanced output, including the part of speech and relationships to the submitted word.
The Big Huge Thesaurus API provides a convenient interface for initiating a search based on an English word and retrieving related words and phrases.
The Big Huge Thesaurus includes more than 145,000 english language words.