I do not typically go to this source for pastorally useful definitions, but my recent post led to me do a little bit of research to see if there are any good definitions of pornography out there.  The Catechism of the Catholic Church actually has about as good a statement as any I have come across:

2354: Pornography consists in removing real or simulated sexual acts from the intimacy of the partners, in order to display them deliberately to third parties.   It offends against chastity because it perverts the conjugal act, the intimate giving of spouses to each other.  It does grave injury to the dignity of its participants (actors, vendors, the public), since each one becomes an object of base pleasure and illicit profit for others.  It immerses all who are involved in the illusion of a fantasy world.  It is a grave offense.  

This seems to me to be pretty thorough and the kind of statement which also repays careful pastoral reflection, as it highlights numerous ways in which the phenomenon is damaging to all concerned.