The order of a function is the nesting depth of function arrows:

• Order 0—values, for instance, 1, false or "hello"
• Order 1—functions A => B, for instance def addOne: Int => Int = x => x + 1
• Order 2—higher-order functions A => B => C, for instance:

def map: Vector[Int] => (Int => Int) => Vector[Int] = 
                                        xs => f => xs.map(f)

• Order 3—higher-order functions A => B => C => D

Any function of an order strictly greater than 1 is a higher-order function.