A function is a block of organised, reusable code that is used to perform a single, related action. Functions usually "take in" data, process it, and "return" a result.
A function defined by user.
Example in PHP
//Function definition
function add ($a, $b)
{
$sum = $a + $b;
return $sum;
}
//Function invokation
$sum = add(1,2);