Defense is a linear trait. Each point of defense gives 0.352% damage reduction from enemy attacks. Written other ways, 10 points of defense will give 3.52% reduction, and 100 points will gives 35.2% reduction.
Therefore, it takes 284 defense to get 100% damage reduction.
Legend bonuses can give 5% more and Perks can give 28%. These percentages are already applied to the base defense on the character sheet, and are pre-applied to armor (that is, they show up in the armor's defense value on the gear selection page).
Gladiator's Coin can also give a bonus of 38 (?) defense when fully levelled.
With the Iron Sword (14), Barbed Carapace (113), Titan Pauldrons (71). and Soldier's Cap (43), plus the Defender skill (28) and Base defense (28), and Gladiator's Gift(38) it is possible to get 335 defense or 125% reduction. What the purpose of having over 100% is... (?)
Note: The highest damage in the game from a monster is the White Sasquatch which has a damage of 70. The next closest monster does 18 damage. For this purpose, it is the best monster to test defense against, as it will give the most accurate number.