A fairground is the open land where a fair happens — the kind with rides, games, food stalls, and bright lights, not the “be fair to each other” kind of fair. You walk on it; the rides sit on it. The word names the place, not the fun itself: after the trucks pack up, the fairground is just empty ground again. In British English this is the everyday word. In American English people often say the fairgrounds, with an s, and they may mean a big permanent site where a county fair or show comes back each year. Though actually, speakers also use fairground for the whole noisy world of rides and stalls, as if the place and the event were one thing. If you only wanted the event, fair would be enough; fairground keeps pointing at the ground under the lights.