Toughness

Toughness is a Fallout, Fallout 2, Fallout 3, Fallout Tactics, Fallout: New Vegas, and Van Buren perk.

Fallout, Fallout 2 and Fallout Tactics
When you are tough, you take less damage. Each level of this perk adds +10% to your general damage resistance.

Fallout 3
With the Toughness perk, you gain +10% to overall Damage Resistance, up to the maximum of 85%. You will also gain a unique dialogue option when talking to Ted Strayer during the You Gotta Shoot 'Em in the Head quest.

Fallout: New Vegas
With the Toughness perk, you gain a permanent +3 to your Damage Threshold; if you take the Toughness perk a second time, this benefit increases to +6. This perk is useful if you take the Kamikaze trait as it allows you to gain 10 AP and 1 DT.

Van Buren
Even tougher than the old Toughness! No longer just bullets and punches, now you can resist fire, plasma, and grenades!

Behind the scenes
The image of this perk is a reference to Superman. In fact, in the perk image for Fallout 1, Fallout 2 and Fallout Tactics, the Vault-Boy character sports Superman's hairstyle. Perhaps fearful of copyright hassles, for Fallout 3 developer Bethesda changed Vault-Boy's hairstyle to its more familiar "blond" appearance, while retaining every other element of the original image. The image in that fashion was carried over into Fallout: New Vegas, most likely due to the fact that the Obsidian development team were largely using Bethesda assets wherever possible.