The value of the largest observation in the lowest 50% of a data set. 50% of the points in a data set are equal to or less than this value. It is also called the 'second quartile', 'median' or '50th percentile'.
There are three quartiles, dividing the data set into four equal parts. They are shown below for a data set with 10 values: 49, 15, 3, 51, 12, 26, 60, 12, 17, 7.
For an even number of values, Q2 is the average of the middle two values in the ordered data set. For an odd number of values, it is the middle-most value in the ordered data set.