The Free Dictionary: Medical Dictionary - The Free Dictionary's main source is is The American Heritage® Stedman's Medical Dictionary, Second Edition, which provides authoritative descriptions of medical conditions, medications, anatomical terms, noted medical personalities and much more.
MedTerms Medical Dictionary - online medical term dictionary produced by MedicineNet, Inc., an online, healthcare media publishing company. MedicineNet is 100% Doctor produced.
MEDLINEplus Health Information - links to online medical dictionaries and lists of online medical dictionaries, produced by the National Library of Medicine.
Merriam-Webster Medical - search medical terms and abbreviations; includes audio pronunciation.
WebMD - 'Find definitions for thousands of medical terms, treatments, and tests -- even health-related abbreviations, prefixes, and suffixes.'