1. the value such that one part of the numbers are larger, and the rest part are smaller.
2. the value of the boundary at the 25th, 50th, or 75th percentiles of a segment divided into four parts, each containing a quarter of the population .
Note: the values at the 25th, 50th, 75th percentiles are called lower quartile(first quartile), median and upper quartile(third quartile) respectively.
see box plot