A mainframe is a very large, very strong computer that many people can use at the same time. Banks, airlines, and governments still run their central records on them. It is not the tall box under a desk, even a powerful one, and it is not a laptop. The name comes from the big metal frame that used to hold the machine in a special cold room. Though actually, the machines today look different, but people kept the word. Workers in computer rooms say it without smiling; to everyone else it sounds a bit old, like something from a film about the 1970s, which is fair — that is when they ruled — but they never really went away.