A second-order simulacrum is a symbol that doesn't actually represent anything. The concept of 'the number' is a representative symbol (as in, an abstraction) of all numbers -- but isn't actually itself a number; it is a second-order simulacrum.

The proverbial average household with its whitewashed picket fence and that poor .5 of a kid (It's a damned shame what happened to poor .5!Timmy. People should be more careful around asphalt crackign equipment and rutting mules.) is a second-order simulacrum used as a 'conceptual placeholder' to make discussing households far simpler.

https://secure.wikimedia.org/wikipedia/en/wiki/Second-order_simulacra