Composition (also referred to as comp) describes the percentage of a demographic segment that is found within the audience of a particular medium---online, application, magazine, newspaper, radio, TV, etc.
For example, the audience of imaginary site QRS.com is comprised of 6,000 women aged 18-34, 39,000 women aged 35-54, and 5,000 women 55+. The total audience of imaginary site QRS.com is 50,000.
The composition of women 18-34 on QRS.com is 12%. The composition was calculated by dividing the number of women aged 18-34 (6,000) by the total audience of QRS.com (50,000.)
In this example the composition of women 35-54 is 78%, and the composition of women 55+ is 10%.
See also Composition Index.