Defence Skill measures how agile you are at evading hits; in other words, it's like Armor Class from D&D. For both melee and missile combat, Defence Skill is what prevents the full force of a hit (or the entire hit) from landing.

In melee combat, Defence Skill opposes Attack Skill in a duel of DRN rolls. In missile combat, it duels with Precision. In both cases, the Defense roll wins in the event of a tie.

Defence Skill is all-or-nothing, really; for reducing damage if the hit lands, you need Protection or an appropriate Resistance.

Defence Skill also comes up in a few niche cases, imitating a Reflex save from Dungeons & Dragons. The big ones are avoiding getting Trampled and avoiding being eaten in an Earthquake; in these cases, a 3d6 roll (3 to 18) is taken, and the target's Defence Skill has to be higher than the result for them to avoid it.

A unit's Armor, if significant, typically reduces Defence. A unit's Shield also reduces Defence; however, the Shield provides a Parry bonus that offsets this. The Parry bonus is added to Defence Skill for melee and missile combat (but not for those niche Reflex cases). If the target's Defence roll succeeds at beating the attacker's Attack roll, but only because of the Parry bonus, the hit is blocked by the shield. The shield's Protection is then added to whatever Protection the target has.

Fatigue, a measure of how tired-out a unit is, also reduces Defence. Every 10 points of Fatigue counts as -1 to Defence Skill.

Lastly, Defence Skill decreases due to "Harassment", a mechanic that simulates units getting cornered or otherwise overwhelmed by multiple attacks in a short succession.

Some weapons provide a Defence modifier (positive or negative); however, this only applies to melee combat. A guy with a Main Gauche of Parrying does not suddenly get better at avoiding or blocking arrows, nor does he improve at running out from an Elephant's path.

This list will not include the Parry bonus, since the Parry bonus does not prevent attacks from hitting.