Discourse on defense

 Combat offense in D&D is modeled with attack rolls, using d20, modified by class and level plus ability score modifiers or by creature Hit Dice, in either case representing some combination of skill and physical power, and by a damage roll, generally representing the amount of force brought to bear on a successful attack. It can be a wonky system, and sometimes counter to our intuitions of how combat and bodily injury actually work, but it serves its purpose well enough in the game.

Defense in D&D combat can be even wonkier and less intuitively satisfying. Like offense, it's broken down into two primary components, Armor Class and hit points. Both, conceptually, serve the same broad function, which is to represent the avoidance or reduction in severity of bodily injury, but they do so in ways which are mathematically distinct and very different intuitively. 

Armor Class is the more intuitively comprehensible of the two, though it itself represents two semi-distinct concepts. The first is the outright avoidance of blows, what we might term "dodging," or "deflecting" and associate closely with a character or creature's agility or Dexterity ability score. The second is to decrease the damage of a blow, either dispersing its energy over a greater area or spreading it over a slightly longer interval of time (recall that force = mass times acceleration; decreasing acceleration, such as with soft padding, thus decreases the force.) This facet of defense is commonly associated with body armor. Armor Class generally takes no account of a combatant's class, level, or Hit Dice; it is a function of agility and of the toughness and coverage of armor or hide; thus we can safely assume it does not incorporate fighting skill. One of the most bizarre aspects of AC is that the former actually seems to count for much more than the latter in most cases. An unarmored human or demihuman character has a base AC of 9, which equates to a 55% chance of taking damage from any given attack by a 1st-level attacker, meaning ordinary agility and standard clothing combine to provide essentially 45% protection. Plate armor, with its standard AC of 3, equals 6 points of armor, or 30% protection. Granted, that 30% is always added to the base 45%; you'll never see a character with only the 30% protection of plate armor, so it wouldn't quite be correct to say that unarmored is "better" than plate armor, but it still feels quite wonky in the think-meat. 

Hit points, in contrast to AC, absolutely do incorporate a character's combat skill, as it relates to class and level. A 5th-level fighter generally has a lot more hit points than a 1st-level fighter or a 2nd-level magic-user. Though in real life, of course, combat ability has a large bearing on a combatant's ability to avoid damage completely, in D&D, it seems combat ability correlates primarily to the ability to reduce the severity of bodily injury. Where the static AC number primarily describes the total avoidance of blows or the dispersal of energy, hit points are much more representative of redirecting force. This can be in the sense of pure physics, i.e. turning a perpendicular force by 45 degrees, thus reducing impact by about half, or by diverting impacts away from very vulnerable areas toward less vulnerable parts of the body. Thus, a 1st-level fighter might take a blow of 8 hp damage as a lethal stab to the heart, slash across the throat, or crushing blow to the skull, but the superior skill and honed combat reflexes of a 4th level fighter might turn that 8 points into a shallow cut, a puncture to fleshy tissue, or painful contusion by either partially redirecting the blow with a weapon or shield or changing the position of his own body in relation to the incoming blow.

Hit points arguably also represent sheer physical toughness rather than skill (especially in the case of Constitution adjustments) and to a certain extent I believe they do, but I tend to take Constitution to represent stamina, the ability to fight without tiring, and thus maintaining the full effectiveness of one's fighting skill and reflexes much longer, than the pure capacity for absorbing punishment without dying. (Though I had not previously considered it, this actually provides a strong argument for the highest levels of Con adjustments being reserved to the fighter classes in AD&D, as the fighter has a lot more combat skill to lose through fatigue than does, say, the magic-user or thief. A magic-user with an 18 Con might indeed be a better marathon runner or shrug off a case of the flu more easily than a fighter with a 16, but he's not better at keeping himself alive in a fight!) 

Despite their similar broad functions, AC and hp serve in different capacities with respect to their game impacts. Armor Class is very much a moment-to-moment defensive stat; it reduces the odds that you'll take damage from any given attack roll, but the outcome is binary: either you take damage or you don't. Hit points, especially as characters gain levels, help to spread the impact of combat out over time; they are a granular measure of attrition. 

In real life, of course, these two components of defense are not as distinctly separate as they are in the game. Fighting skill plays a role in avoiding blows entirely, and armor plays a role in reducing the severity of injury when a blow does land. This is a game, though, and the parts have to be usable at the table, which usually means a degree of abstraction and simplification. It's certainly possible we could construct a different model of combat defense and survivability, but what we have has worked for decades. At the very least, we ought to understand as fully as possible how the pieces function and interact so if we choose to modify the system or create something new from scratch, we'll have an idea what its impact will be on the feel and playability of the game.