Extreme, sadistic, over-the-top bloody violence. A portmanteau of "gore" and "porn". The term can refer to just an extremely graphic scene of bloodshed, or the entire sub-genre of torture films in the spirit of Saw.
Gorn shows gore and blood, not to frighten or upset, but to excite and thrill. Like pornography in which a flimsy plot about a pizza delivery boy and a spaceship's randy, all-female crew is simply a device to get people to take their clothes off and have some sex already, the gorn genre is also all about "getting to the good stuff"... though that isn't to say that all gorn is mindless drivel. Just like porn.
Certainly, horror relies on placing its characters in terrible situations so that the audience can experience the shock, even the thrill, of their peril. However, gorn isn't about the audience enjoying a good scare. Gorn isn't concerned with the protagonist's struggle to survive. Gorn exists simply for the sheer enjoyment of a plain old bloodbath.
The environment version of this trope is Scenery Gorn.