Do you ever see 99.9% or the “five nines” – “99.999%”bandied about? Its usually in relation to uptime – how much uptime or conversely downtime your service is up or down for. Pretty important when you want to determine how reliable your service provider is likely to be when you might be hosting your entire business in the cloud or similar.
Be aware that when you are quoted an uptime figure – the definition may vary from provider to provider – for example some providers will not include “scheduled downtime” – for reboots, updates etc – so make sure you are certain what is covered and if you are comparing oranges to oranges!
A handy website that help you calculate what a particular % up time means for your downtime!
A service level agreement will often use a % uptime to explain the service commitment.