What is a Hit Point?

Now, you really can't discuss any particular facet of D&D combat (such as Hit Points) without touching on the other aspects of that abstraction Armor Class/Defense, Saves, Critical Hits, Death and Dying Mechanics, etc.

The way I run combat, Hit Points are not a definable quality of a being.  They are purely a game mechanic and not diegetic to the game world.  Most importantly, the loss of Hit Points does not equal a wound.

Taking a step back, Hit Points (and Saves) are things that increase as characters level up (or creatures have greater Hit Dice).  Compare this with Armor Class, a 1st level warrior can easily put on plate mail, and have a great AC, but there is little they can do to improve HP or Saves. This makes Hit Points/Saves a reflection not of a naturalistic quality of the being, but more a measure of their "value" in the game.  To borrow a term, they are a mechanical representation of "plot armor".

Now, this approach to HP does have some wrinkles:

• If HP are not diegetic, then what does the Cure Light Wounds spell represent?
• I get it, it is right in the name of the spell.  I generally hand wave this as a blessing to renew vigor, almost a divine shot of adrenaline.  I don't go so far as to rename the spell, just sort of keep the above definition in mind when describing things narratively.
• If a "HIT" does not always equal the wounding of a creature, then how to you reconcile add-on effects (Poison, Ghoul Paralysis, Bleeding, etc.)
• To start, this is one of those areas that as the game evolved, folks took the abstraction of Hits and Hit Points, and treated them more literal.  Not much can be done about that.
• However, most of those effects have some sort of Save or confirmation mechanic.  As such, we can use the outcome to retroactively describe the "Hit".  Passed the Save? Then the blade narrowly passes by your flesh, the poison dripping down on your skin.  Failed? then the Ghoul's claws pierce through the seams of your armor, allowing the necrosis into your bloodstream.
• Critical Hits, I will often describe these as a strike or wound.  Like when Rocky gets his first real hit on Drago. It rewards the excitement of the Crit.
  

In the end, I think this lack of solid definition of Hits, HP, etc. is a feature, not a bug.  It allows a hit to be a wound when it needs to be, it allows for luck or awesomeness when needed. It is a great tool for robust game play and narrative.

Below are my general guidelines with regard to Hit Points.

• Hits and Hit Point loss are not wounds, expect when it is obvious they are.
  
• When creatures get down to their last bit of HP (~ CON score + HD/Level) , they are "bloodied", so I will start describing cuts, bruised, etc.
• I will describe a "non-wound" HP loss as a wearing down of ones defenses, pushing back, etc.
• Injuries (Scars, etc.) don't really happen until one reaches 0HP (and/or perhaps Critical Hits, depends on the system).
• HP based healing will often be described as renewing of vigor, easing of aches.  Unless they are down in the above described "bloodied" range, in which case, bruises clear, cuts close, etc.