p99 Latency
also: tail latency · percentile latency
The response time your worst 1% of requests exceed — the tail users actually feel.
p99 latency is the 99th-percentile response time: 99% of requests are faster, 1% slower. Averages hide this tail, so SLOs are written against p95/p99. Because users touch many services per action, one service’s tail becomes the common experience.