Matrix numbers (I've also heard the term batch numbers) are terms used to describe the numbers that you see on the back of CDs in small print. There are two different kinds of matrix numbers. There are numbers printed on the foil layer, and there are numbers carved or stamped into the plastic itself. They are both found around the inner ring of the CD .

Since many compact discs are often re-released with slightly modified contents (e.g. particular tracks edited, or hidden tracks moved or omitted), it is often useful to know which pressing of a compact disc you have, and the catalog number and UPC don't always change with a re-print.

They are also useful to find out if the CD you are about to buy is copy protected

They are named after the numbers that have been historically stamped into vinyl records. Explanation of Matrix Numbers is a good article about matrix numbers on records.