Medium

1. Of intermediate size, degree, amount, etc. He wears a medium size shirt.
2. Of meat, cooked to a point greater than rare but less than well done; typically so the meat is still red in the center. He likes his steak served medium rare.
3. An intervening substance through which something else is transmitted or carried on. Water is a favorable medium for biological growth.
4. An agency by which something is accomplished, conveyed or transferred, The train was a common medium of transportation in the 1930s.