Extensionality is a principle in set theory that states two sets are considered equal if they contain the same elements. This means that the identity of a set is determined solely by its members, not by how it is defined or constructed. For example, the set 1, 2, 3 is the same as the set 3, 2, 1 because both contain the same elements. In logic and mathematics, extensionality helps clarify the concept of functions as well. Two functions are deemed equal if they produce the same output for every input. This principle is fundamental in various fields, including mathematics, computer science, and philosophy.