Uptime
The percentage of time a system is operational. Counted in nines: 99.9% allows nine hours down a year; 99.999% allows five minutes and requires a much larger budget than the extra digits suggest.
Heard in the wild
“The SLA guarantees 99.9% uptime, with credits if they miss.”
Tone check: Technical
Each additional nine costs roughly 10x. Ask what the business actually needs before paying for five of them.