Despite what it might sound like, an airfield is not a field somewhere in the atmosphere above ground-level. If, based on the word’s constituent parts, air and field, that’s what you thought it meant then I strongly recommend that you consider honing your critical thinking skills, right after you get some.
What the hell would a field in the air even be? What purpose would it fulfill? It’s not as if you could play a game of football on it or let your dog run free on it. Heavier-than-air objects, such football players, footballs and dogs, have a strange tendency to fall toward the earth. It’s called gravity and it would make a field in the air pretty much worthless for most purposes. Use your head a little before jumping to a conclusion regarding the meaning of the world airfield.
Instead, an airfield is a field (the word field being used loosely to mean a suitably large area) that includes a landing strip for aircraft.
When I say “landing strip” you can assume that I mean it can be used for airplane takeoffs too. An airfield that allows only landings would get overcrowded and have to shut down when it ran out of room. What’s more, airplane owners would likely become a tad annoyed if they had to abandon their planes there.
If it has a lot of terminal buildings—or even just one decent-sized terminal building—used by commercial aircraft you usually don’t call it an airfield, but rather an airport.
It’s not technically inherent in the definition, but if you’re talking about non-military airplanes at airfields rather than at airports you probably won’t go far wrong thinking about small, private craft that don’t have room for more than a handful of people. This assumes, of course, that the appendage belongs a mythical creature with a hand large enough to accommodate no more than a few people.
Military airports are often called airfields regardless of the size of the planes flown in and out of them and regardless of whether or not there are any large terminals there.
That might be confusing because of the contrast with non-military airfield definition, but tough. It’s the military. What are you going to do? Fight with them about it? I wouldn’t advise it. They’ve got big guns.