A pure function is a type of function in programming that always produces the same output for the same input, without causing any side effects. This means that when you call a pure function with specific arguments, you can expect the same result every time, making it predictable and reliable. Additionally, pure functions do not modify any external state or variables. They do not change data outside their scope, which helps in maintaining the integrity of the program. This characteristic makes pure functions easier to test and debug, as they operate independently of the program's state.