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1. The percentage of a distribution with a lower score or rank. For example if 70% of the population were shorter than you, then your height would be said to be at the 70th percentile. See also quintile, decile.
2. The percentage of a distribution with an equal or lower score or rank, i.e. similar to the above but "equal or lower" rather than "lower".

For more information see:

  • http://cnx.rice.edu/content/m10805/latest/ - briefly covers both definitions given above plus an alternative approach which uses weighted averages and guarantees the median to be the 50th percentile. It also provides a method to calculate a percentile when the data might not otherwise seem to naturally fit, for example the 34th percentile of four numbers.