Thorns refers to any sort of item or property that reflects damage back at the attacker. This modifier is seem on numerous items in games like Diablo 3, and there are usually monsters with the power as well.
In Diablo 3 the boss monster modifier Reflects Damage is a prime example, and many players have killed themselves while winning a battle against these enemies. Properties that heal the player, such as Life on Hit or Life Steal, or skills that have the same effect, such as Gloom, are essential to survive battles with monsters with "RD."
Player equipment can spawn with thorns properties as well, but through the first year of Diablo 3 these were considered useless, as the damage they reflected was miniscule compared to the hundreds of thousands or millions of hit points possessed by Inferno level enemies. The developers agreed with that assessment and gave a hint of the changes that might be coming up, in a post by Travis Day in March 2013.