Aerospace is the business of designing and building machines that fly in the air or go into space: aircraft, rockets, satellites, and the tools around them. People use it as a name for the whole industry — the companies, the engineers, the jobs, the money. They also set it in front of other words: aerospace engineering, an aerospace firm, the aerospace sector. It is a formal, grown-up word; you will meet it in news and in science, not in the playground. Aero- points to the air; space points to what is beyond the air. The two were pushed together in the twentieth century, when flying and space travel began to share the same workshops and the same science. Though actually, a paper plane is not aerospace, and neither is a city bus: the word stays with those machines, and with the industry that makes them.