Uptime

Uptime is the share of time that a system, such as a website or an online service, is working and available for people to use. It is usually given as a percentage over a period, so high uptime in a month means only a few minutes of unplanned outage.

It is like a shop's opening hours actually being honoured: customers who turn up find the doors open rather than a closed sign. Uptime matters because every minute a service is down can mean lost sales, stranded staff and a dent in the trust customers place in you.