The notation "S_n" typically refers to the symmetric group of degree n, which is a mathematical concept in group theory. It represents the set of all possible permutations of n distinct objects. For example, if n = 3, then S_3 includes all the ways to arrange three items, such as 1, 2, 3. The symmetric group is important in various fields of mathematics, including algebra and combinatorics. It helps in understanding the structure of permutations and their properties, such as composition and inverses. The size of S_n is given by n!, which is the factorial of n, representing the total number of permutations.