Helpless

Helpless is a state applied to creatures victim of certain status effects. It is often mentioned in game descriptions.
Any physical attack against a Helpless defender will not be required to go through an attack roll in order to land a blow: The to-hit calculation is bypassed and the defender's Armor Class ignored. Physical attacks therefore always result in a normal hit — without an attack roll, Critical hits and misses are impossible. 
Defences other than Armor Class and penalties to attacker THAC0 do still apply, such as Protection From Magical Weapons, Mirror Image, Stoneskin, damage resistance, etc.
The following status effects are able to cause the Helpless state:
*Hold creature
*Hold creature 2
*Paralyze
*Sleep / unconsciousness
*Stun
*Web effect
Creatures affected by Time Stop suffer the same consequences as Helpless, but they don't actually get the state proper.
Similarly, a weapon with a natural THAC0 bonus of 32,767 bypasses attack rolls as if the defender were Helpless, even though they are not — it cannot critical hit, cannot miss, but can still bump into defences other than Armor Class. Arrows of Detonation have such a THAC0.
==Sources & references==
*Insight credited to kjeron
Category:Game mechanics
Category:Status effects