There are now many medical dictionaries online, but one of the best is MedTerms, at MedicineNet.com. Here is a description from the site:
MedTerms medical dictionary is the medical reference for MedicineNet.com. Our doctors define difficult medical language in easy-to-understand explanations of over 16,000 medical terms.
MedTerms online medical dictionary provides quick access to hard-to-spell and often misspelled medical definitions through an extensive alphabetical listing.