These online medical dictionaries provide short definitions of medical and health terms, so that you can fully understand what you are reading.
Online Medical Dictionary
Merriam-Webster Medical Dictionary
Medterms Medical Dictionary
Need a bit more detail than a definition? Try these free online sources
Medline Plus Medical Encyclopedia
Wikipedia is an online free-content encyclopedia. Keep in mind that Wikipedia entries vary in quality. Resources cited in the bibliography of an entry could also lead you to helpful resources.