It's a long way down.

This is a quote from a talk at the International Conference on Function Programming (or more specifically the Workshop for Commercial Users of Functional Programming http://cufp.galois.com/2009/abstracts.html). But Lee Benfield who said it, attributed the saying to Simon Peyton-Jones.

A higher-order functional language allows higher order functions like map, filter and foldl (functions on functions). A first order language allows to define functions, but not on other functions. And a zeroth order language does not allow you to define functions (or name anything) as far as I figure.