The median is the middle number for a set of data when the data are arranged in order from least to greatest or greatest to least. To find the median of a set of numbers, arrange the numbers in order and then find the middle number. 

Example: when there is a middle number 
The median is simpler to determine when there is a middle number in the set. 
Given this set of numbers (arranged in order): 1, 3, 6, 9, 12 
Median: 6 


Example: when there is no single middle number 
If there is no single middle number, the median is the arithmetic mean of the two middle numbers. 
Given this set of numbers: 1, 2, 4, 5, 7, 8 
(4+5) ÷ 2 = 4.5 
Median: 4.5 
