Originally Posted by Caine
Get yourself an instant read thermometer. Insert it into the exact center of the steak at a 45 degree angle.
Rare = 120F (49C)
medium rare = 126F (52C)
medium = 135F (57C)
medium well = 145F (63C)
well done = 160F (71C)
This is somewhat subjective, and it may depend on your thermometer and how you insert it, but in my experience, 120 is medium-rare for beef, particularly once the meat has been allowed to rest.
I don't find thermometers to be useful with most steaks, which are too thin for an accurate reading, but I do like to use them for thicker cuts of beef, say 2 inches or more, and for things like butterflied leg of lamb and whole chickens.