A permanent loss of 1 or more ability score points. The character can regain these points only through magical means. A character with Strength 0 falls to the ground and is helpless. A character with Dexterity 0 is paralyzed. A character with Constitution 0 is dead. A character with Intelligence, Wisdom, or Charisma 0 is unconscious.
If a creature has an attack that deals ability drain, the creature's descriptive text gives the ability and the amount drained. If an attack that causes ability drain scores a critical hit, it drains twice the indicated amount (if the damage is expressed as a die range, roll two dice). Unless otherwise specified in the creature's description, a draining creature gains 5 temporary hit points (10 on a critical hit) whenever it drains an ability score no matter how many points it drains. Temporary hit points gained in this fashion last for a maximum of 1 hour.
Some ability drain attacks allow a Fortitude save (DC 10 + 1/2 draining creature's racial HD + draining creature's Cha modifier; the exact DC is given in the creature's descriptive text). If no saving throw is mentioned, none is allowed.