Percentile

P10 means 10% of the samples.

For example, 0.5ms at P10 that means 10% of our samples is <= 0.5ms

Similarly, we have P90, P99, P50, …a

A median is p50 (50 percentile). i.e Take all the datapoint, sort it, take the middle one. That's the median – 50 percentile

To calculate the percentile, we have a few option

Group all the data within the time window and sort the data
Use some algorithm can calculate good approximation i.e forward declay, $t-digest$ or $HdrHistogram$

[!danger]
For percentile we cannot take the average, for example, if server A P90 is 200ms, server B P90 is 150ms. We cannot do server A and B P90 = 350ms/2