From ErfWiki

[edit] Proposed Canon

Defense is a Unit statistic.

[edit] Known Values

• Bogroll - 4
• Jack Snipe - 2

[edit] Speculation

It probably determines how hard it is to score a hit on this unit, provides resistance that reduces inflicted damage, or is a combination of both. In other words, it probably affects a unit's chance to survive wounds and attacks in combat.