A permutation is an arrangement of items in a specific order. For example, if you have three letters, A, B, and C, the different ways to arrange them are ABC, ACB, BAC, BCA, CAB, and CBA. The order matters in permutations, meaning AB is different from BA. Permutations can be calculated using a formula. For n items, the number of permutations is n! (n factorial), which means multiplying all whole numbers from n down to 1. For instance, with 3 items, the calculation is 3! = 3 × 2 × 1 = 6 permutations.