Food you get at a restaurant where you order at a register, go sit down, and have it brought to you. Generally faster than a sit-down meal, but better for you (and better-tasting) than fast-food. Examples include small Italian restaurants, Fuddrucker's, and many Asian lunch joints.
Medium food also includes restaurants where you order at the counter, go sit down, and are called back up to get your food a few minutes later.
Mike: Dude, let's hit Wendy's for a quick lunch.
Dave: Nah, I'm trying to lose weight. No fast food. How about medium food?
Mike: Sure, that burger joint on Main Street has a special today. Let's go!