A function is a named piece of code comprised of one-to-many statements to perform a specific task. A function is called multiple times in a single script to repeat its specific task. A parameter is a named type value available as a variable within the statement(s) of a function. A function specifies zero-to-many parameters, and when a function is called a value is specified per parameter. An argument is a value passed into a function at the point of call. A function specifies a return type value, though if the type is void then no value is returned. Any non-void type return value is available for use within an operation or is discarded otherwise.

You can declare functions at the beginning of a Painless script, for example:

boolean isNegative(def x) { x < 0 }
if (isNegative(someVar)) {