It stands in contrast to the concept of intensionality, which is concerned with whether the internal definitions of objects are the same.
The extensional definition of function equality, discussed above, is commonly used in mathematics.
Extensionality principles are usually assumed as axioms, especially in type theories where computational content must be preserved.
Similarly, in natural language there are many predicates (relations) that are intensionally different but are extensionally identical.
Then, the two predicates "being called Joe", and "being the oldest person" are intensionally distinct, but extensionally equal for the (current) population of this village.