What is first class function in Python

Question

I am still confused about what first-class functions are. If I understand correctly, first-class functions should use one function as an object. Is

Answer 1

def pie(r):
    def circleArea(d):
        return r * (d ** 2)
    return circleArea

c = pie(3.14)
print c(2)

Above is an example for first class function in python.