Tail latency
Also: p95, p99, percentile latency
The latency at a high percentile: p99 is the figure 99% of requests come in under. It describes the slow requests that shape how a service is perceived.
What people get wrong
Averages hide it entirely, and averages of percentiles are meaningless. Report p50, p95 and p99, and never take the mean of p99 values across time buckets.
A definition is not an understanding. This is taught properly in The distributions you will actually meet, part of Probability & statistics.
Learn this properly