Wordnik is a non-profit English dictionary aggregation service founded by Erin McKean (former American Heritage Dictionary editor). The API aggregates multiple licensed or open-source dictionaries into a unified interface: word definitions (sourced from American Heritage, Wiktionary, multi-source), synonyms/antonyms (WordNet), etymology, example sentences (from real-text corpus), pronunciation (phonetic + audio URLs), related words, variants. Free API keys work well for personal and educational projects. vs. Oxford Dictionaries API (paid, single-authority): Wordnik's multi-source aggregation has broader coverage (especially new words, slang); downside is occasional inconsistencies between sources. Word of the Day widget is embedded in many English learning apps.