quantiles

The quantile (this term was first used by Kendall, 1940) of a distribution of values is a number x_p such that a proportion p of the population values are less than or equal to x_p. For example, the .25 quantile (also referred to as the 25th percentile or lower quartile ) of a variable is a value (x_p) such that 25% (p) of the values of the variable fall below that value.
Similarly, the .75 quantile (also referred to as the 75th percentile or upper quartile) is a value such that 75% of the values of the variable fall below that value and is calculated accordingly.
See also, Quantile-Quantile Plots