👉 The term "derangement" refers to a permutation of a set that does not contain any elements. In other words, it is the number of ways to arrange a set without any element in common with another set. For example: - The derangement of {1, 2, 3} is 0 because there's no way to put them together. - The derangement of {4, 5, 6} is 18 because it can be arranged as {4