A room, shop, bus, or whole building is air-conditioned when it has a system that makes the air cooler and drier, so the space feels less hot and less sticky. Though actually, air-conditioned is the word for the place that has that system; the system itself is usually called air conditioning. In everyday talk people ask 'Is it air-conditioned?' and mean 'Is it cool inside?' It is a plain, useful word — a little more careful than just saying 'AC'. In very hot weather it is not a fancy extra; it is how offices, malls, and some classrooms stay bearable. You can also hear it of cars and trains.