Availability describes how dependably a service can be reached and used, usually stated as a percentage of time it is operational. In practice it is used interchangeably with uptime.
Availability targets are written as “nines” — 99.9% (three nines), 99.99% (four nines), and so on. Each additional nine cuts allowed downtime roughly tenfold.
Availability is what an SLA promises and an SLO targets internally.
