What are Hit Points?

In video games, Hit Points (HP) are a pretty static concept. They equal "health" and are an indicator of biological vitality. It's well-accepted that when a character's HP drop to zero, that persona is dead (or possibly comatose until revived via extra lives or a "continue"). But for sake of argument, that character is gone. Game Over.

 It could also mean "energy" though. Whether that's a metabolic quantifier or some other esoteric measure. When the character runs out, HP needs a recharge! Then it's back into the fray. So does that mean HP should be decreasing over time in traditional RPG--like when we need sleep?

 In pen and pencil games, HP can get--let's just say--squishy. It can represent a few things or a combination of things. Some games go for the straight-up health stat, while others make it a loose representation of "life" and uh--how do I put this--resistance to being hit. Yeah, like "something" gets in the way of that well-aimed laser and our hero is hit, but for some reason a blast only does 2d6 damage and our guy has a crazy amount of hit points. How did did get those points anyway? Did he have armor? Of course!

But that was taken into account with armor class (AC). Was it his dexterity/agility that allowed him to dodge? Well if it was--then he'd have escaped being hit--and at least in this example, he took the hit. His constitution/physique? Again, those scores are already taken into account.

Wouldn't make sense to have both HP and constitution (even though in many games HP is determined by constitution or some similar stat)? What's the point of representing a stat made up of other stats? (Don't answer that one--not yet!) In some games (and sometimes its just the GM's preference) rely on XP to be the "x" factor.

The more experience under your belt, the greater your HP to protect yourself. Makes sense in a way, right? Your character has managed to live this long--a little extra "fortitude" (whatever that means) is a reward to help protect yourself as you advance onto tougher challenges and meaner foes. Except that's the exact opposite of how real life--and combat--works. If anything, more injuries will add up over time. And the aging process guarantees greater frailty.

  So what are Hit Points? How are you using them? As proclaimed in "the rules"? Do you let players trade them in order to accomplish difficult or impossible tasks, otherwise beyond their reach? To the comments!