What is Percentile?
A percentile is the value below which a given percentage of the data falls; the 90th percentile is the value that 90 percent of observations do not exceed.
Percentile explained
Percentiles describe the shape of data better than a single average. The 50th percentile is the median; the 90th or 95th shows what the slow end looks like, which is how service-level targets are often written.
Databases calculate them with functions such as PERCENTILE_CONT. Quartiles are the 25th, 50th and 75th percentiles.
Example
If the 90th percentile of delivery time is 4 days, nine in ten deliveries arrive within 4 days.