What counts as "Thorns" damage for the barbarian "Tough as Nails" passive ability?

Question

The "Tough as Nails" passive ability says:

Increased armor by 25%

Thorns damage dealt increased by 50%.

I'm not aware of a specific ability called Thorns in Diablo 3, so I assume this relates to gear that causes damage to melee attackers, but am I right? Are there other sources of "thorns" damage?

Answer 1

Thorns is short for "damage done to attackers when hit by melee attacks", so you're correct. Any equipment or skill* that deals damage to melee attackers counts as Thorns.

The text you want to look for is "Melee attackers take XXX damage per hit."

This shield for example has 26 "Thorns":

You can see your effective Thorns under the Details tab in your inventory.

_{*I'm unaware of a Barbarian skill that counts as thorns.}