The GCD, or Greatest Common Divisor, is the largest positive integer that divides two or more integers without leaving a remainder. For example, the GCD of 8 and 12 is 4, as 4 is the highest number that can evenly divide both 8 and 12. Finding the GCD is useful in various mathematical applications, such as simplifying fractions and solving problems involving ratios. There are several methods to calculate the GCD, including the Euclidean algorithm, which is an efficient way to determine the GCD by repeatedly applying division.