Toughness/Fallout 4

Toughness is a perk in Fallout, Fallout 2, Fallout 3, Fallout Tactics, Fallout: New Vegas, Fallout 4, and Van Buren.

Fallout, Fallout 2 and Fallout Tactics
+10% to basic damage resistance (+30% total). Useful, as protection is always good.

In Lionheart, the equivalent of this perk is called Damage Resistance.

Fallout 3
+10% to the overall Damage Resistance (capped at 85%).

The perk also adds a unique dialogue option when talking to Ted Strayer during the You Gotta Shoot 'Em in the Head quest.

Fallout: New Vegas
+3 points to the player's Damage Threshold (+6 with both ranks).

This perk can neutralize the DT penalty of Kamikaze.

Fallout 4

 * Rank 1: +10 damage resistance
 * Rank 2: +20 damage resistance
 * Rank 3: +30 damage resistance
 * Rank 4: +40 damage resistance
 * Rank 5: +50 damage resistance

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 for this perk is a reference to Superman, especially in Fallout, Fallout 2 and Fallout Tactics when the Vault Boy has the Superman hairstyle.