State the definition of a function?

A function is a special relationship where each input has a single output.

It is often written as "f(x)" where x is the input value.

Example: f(x) = x/2 ("f of x is x divided by 2") is a function, because each input "x" has a single output "x/2":
• f(2) = 1
• f(16) = 8
• f(−10) = −5

https://www.mathsisfun.com/definitions/function.html