Grazing hit

Attacks with weapons score a normal damaging hit either if the attack roll is a natural 20 on a d20, or if the roll + attack modifiers equals or exceeds the target's AC. However, DDO also allows for an attack that missed the AC to still do reduced damage if the d20 roll was high enough, which is called a Grazing Hit.

Players get a Grazing hit on attack rolls of 10-19. Monsters need a different number depending on the difficulty setting of the quest: 17 on Normal, 15 on Hard, 13 on Elite, and 19 on Solo. The Grazing Hit does damage only equal to the base damage of the weapon (such as 1d8 for a longsword) minus any strength penalties. Damage bonuses from feats, strength, magic weapons, or other sources do not apply to Grazing Hits. Grazing Hits won't work if the attacker is nonproficient with the weapon, and PCs can't get them with offhand weapons.

Presently Sneak Attack damage is added to Grazing Hits, although that is at odds with the developer announcements.

Grazing Hits are subject to DR as usual, which can frequently reduce them down to zero damage. There are additional sources of a percentage reduction to Grazing Hit damage:
 * Player characters have the Heroic Durability feat which provides 50% less grazing damage.
 * Holding a shield reduces Grazing Hit damage by 10%, 25%, or 50% for Buckler/Light, Heavy, and Tower shields respectively. The Shield spell counts as a light shield for that purpose.
 * Centered monks get a 10% reduction to incoming Grazing Hits every 4 levels.

Grazing hits have their own combat log message and a special icon for heads-up damage display.

Developer introduction (out of date): http://forums.ddo.com/showthread.php?t=188041