Try this if you want something to download and install
It looks like it's trying to be an everything dicionary, so i'm not sure about the extensiveness of the medical terms.
It does say that 9,000+ medical terms are included:
http://www.linuxsoftware.org/leaf.php3?catid=147
There are several Linux-compatible medical dictionaries aveilable for purchase; prices range from $30s to $60s
Otherwise, there are several free online medical dictionaries available.
Google on "online medical dictionary"
or start at
http://www.medlineplus.gov