Aid consists of binding wounds, starting respiration, administering a draught (spirits, healing potion, etc.), or otherwise doing whatever is necessary to restore life. It ceases immediately on any round a friendly creature administers aid to the unconscious one. Such loss and death are caused from bleeding, shock, convulsions, non-respirations, and similar causes. I don't object to it in principle, though I can't help but wonder why it's necessary. When I was young, this was a widely quoted – and used – rule, though I'd imagine most of us had no idea precisely where it was found.
In each of the next succeeding rounds 1 additional (negative) point will be lost until –10 is reached and the creature dies. When any creature is brought to 0 hit points (optionally as low as –3 hit points if from the same blow which brought the total to 0), it is unconscious. Whereas Volume 1 of OD&D clearly states that hit points are 'the number of points of damage the character could sustain before death,' (emphasis mine), the DMG instead states:
Of the Dungeon Masters Guide has several different sections worthy of comment, but, for the current post, I'm focusing on Gary Gygax's discussion of zero hit points.
