Uptime
The share of time a system is working and reachable. Downtime is the opposite: minutes when users get errors instead of answers.
What it tells you about a developer
A developer who quotes uptime numbers from memory has been accountable for a live service. Owning an uptime target means they measured failure honestly and built habits around preventing it.
Uptime is quoted in percentages, and engineers count the nines. A system at 99.9% uptime can be down about nine hours per year. At 99.99% the budget shrinks to under an hour. Each added nine costs real engineering: duplicate servers, careful deploys, fast incident response. Downtime costs revenue and trust, which is why teams measure this obsessively.