Merriam-Webster vs Wordnik

Merriam-Webster's Dictionary API returns JSON definitions, pronunciations, and audio from the publisher's own Collegiate Dictionary and Thesaurus, plus specialized editions covering medical, learner's, Spanish-English, and grade-banded school dictionaries, each requiring a separate free API key. Keys are issued under a strict non-commercial license capped at 1,000 queries/day/key; using the service in a commercial product requires a separately negotiated agreement with Merriam-Webster.

Wordnik's REST API returns word definitions aggregated from multiple published dictionaries alongside example sentences, pronunciations, related words, and corpus-derived frequency data, documented through an interactive Swagger UI. Every developer gets a free Basic plan (100 calls/hour, with an explicit nonprofit/research allowance) before paid Hobby, Pro, and Enterprise tiers raise the ceiling up to 45,000 calls/hour -- revenue that funds Wordnik's nonprofit mission to document the English language.