Definition: 95th Percentile Measurement is used to determine the volume of traffic exchanged for the transit fee (or paid peering) calculation.
Monthly 5 minute samples are collected and sorted lowest to highest, using the 95th percentile measure to calculate the monthly Internet Transit bill.
This method allows customers to burst above the 95th percentile measure and not have to pay for that supposedly anomalous traffic. Some of the more clever (some would say seedier) ISPs have gamed this system by sending up to 36 hours of burst traffic and then close to zero traffic for over 95 percent of the time, allowing them to use the transit service at minimal cost.