Explain what the term 'higher-order function' means in programming and how it is used.

A higher-order function is a function that can accept other functions as arguments and/or return a function as a result. This approach allows for creating more abstract and flexible code.

In iOS (Swift), higher-order functions are often used for processing collections, asynchronous operations, and creating closures.

Example of a higher-order function in Swift:

func performOperation(_ a: Int, _ b: Int, operation: (Int, Int) -> Int) -> Int {
    return operation(a, b)
}

let sum = performOperation(3, 5) { $0 + $1 }
print(sum) // 8

Here, performOperation takes a function operation as a parameter and applies it to two numbers. This makes it easy to change behavior without duplicating code.