What is meant by Data?

I saw a very interesting piece of code in the SICP. It took quite sometime for me to understand it completely. In fact, the meaning flashed to me today during my morning jog!

In a programmer's perspective, what exactly is data?
This is a very subtle concept. Very difficult to explain.

Data is something that is constructed and behaves in a particular way. i.e, What data is, is determined by how it behaves.
How do you say this programmatically? - Data is that which is defined by constructors and procedures, together with specified conditions that these procedures must fulfill in order to be a valid representation.

Example: consider the notion of a 'pair' in lisp. A 'pair' is a way of glueing two pieces of data. This is how I will probably explain it:

(define z (cons x y))
(car z) => x
(cdr z) => y

We never actually said what a pair was, only that there are procedures cons, car, and cdr for operating on pairs. But the only thing we need to know about these three operations is that if we glue two objects together using cons we can retrieve the objects using car and cdr.
The interesting thing is that any triple of procedures that satisfies the above condition can be used as the basis for implementing pairs. This is one such triple:

(define (cons x y)
(lambda (m) (m x y)))

(define (car z)
(z (lambda (p q) p)))

(define (cdr z)
(z (lambda (p q) q)))

This point is illustrated strikingly by the fact that we could implement cons, car, and cdr without using any data structures at all but only using procedures. This blurs the distinction between 'procedure' and 'data'. Procedure <---> data. Data <---> procedure. Hmm...I need to think.