Uptime is the time during which a computer, a website, or a machine is working and people can use it. It is the opposite of downtime. In computing you meet it most often as a promise or a measurement: a company that runs servers will say '99.9% uptime', meaning that out of all the hours in a year, the service was working for 99.9% of them and broken for the rest. Because of this, uptime is usually spoken about as a percentage or as a length of time — 'our uptime last month was 99.99%', 'the server has an uptime of 200 days', meaning it has been running for 200 days without being restarted. It is an uncountable noun, so you do not say 'an uptime' or 'uptimes'. The word started in computing and factories, and it still feels technical; ordinary people rarely use it about their own laptop. In factory talk it means the time a machine is producing, as against the time it sits stopped for repair. On Linux and Mac computers there is even a command called uptime that tells you how long the machine has been on. Downtime, its opposite, has escaped into everyday English and now also means personal rest time — but uptime has not made that jump, so it stays inside the technical world.