The definition is here quoted from Wired for Intimacy: How Pornography Hijacks the Male Brain by William Struthers, psychology professor at Wheaton College. After examining several different attempts to define pornagraphy, Struther’s states that the Roman church’s definition is the one he prefers:
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. Civil authorities should prevent the production and distribution of pornographic materials.
William Struthers, Wired for Initimacy: How Pornography Hijacks the Male Brain, 29