"Mod 9" refers to a mathematical operation known as the modulus operation, specifically with the number 9. It calculates the remainder when one number is divided by 9. For example, if you divide 20 by 9, the quotient is 2 and the remainder is 2, so 20 mod 9 equals 2. This operation is useful in various fields, including computer science and number theory. In a broader context, the concept of modulus can be applied to any integer, not just 9. The result of a mod operation will always be a number between 0 and one less than the modulus. For instance, when using mod 9, the possible results are 0 through 8.