What are Five-Nines?

Answer 1

Five nines was a slang used during the World War 1 to mean a German 5.9 inch artillery shell during the period of 1893-1918.

Answer 2

Five-Nines refers to a system or service that guarantees 99.999% availability or uptime. It is a measure of high reliability and is often used in the context of telecommunications, data centers, and cloud computing services. Achieving Five-Nines means the system is expected to have no more than 5 minutes and 26 seconds of downtime per year.