The median is a value that divides the distribution of a set of data into two equal halves. Like the mean, the median is an indicator of the location or center of a distribution. If there are an odd number of data points, the values are arranged in increasing order of magnitude (size), from small to large, and the median is the middle-most value in the arrangement. If the number of data points is even, the values are arranged in increasing order of magnitude, but now the median is the mean of the two middle-most values in the arrangement.
Because there are 10 values in this data set (there is not a middle value), the median is the average of the 5th and 6th data points when arranged by size. M = 4.
NIST Statisics Handbook: Process Characterization - http://www.itl.nist.gov/div898/handbook/toolaids/pff/ppc.pdf