I got Stedman's Medical Dictionary for free when I signed up for the AMA...not pocket-sized, but handy enough that I carried it around during the first two years of med school.
Early on during third year rotations, when I was still unfamiliar with many terms, I used something for my PDA. I think it was the Skyscape version of Stedman's. Dorland's is the other big name in medical dictionaries.
By fourth year, unfamiliar words were popping up infrequently enough that I could just write it down and look it up later.