Incorporeal

Incorporeal creatures cannot be easily hit with normal weapons, which suffer a 50% chance of automatically missing the target. Ghost Touch weapons avoid this penalty, as do weapons wielded by somebody wearing Ethereal (Ethereal Bracers, Spectral Gloves), or Ghostly. (Wretched Twilight, level 16 or 20 Cloak of Night.)

There's no such thing as touch attacks in DDO, which is how Incorporeal Undead attack. To compensate for this, developers gives them very high to-hit bonuses to simulate it.

They DO miss, but it takes substantial Armor Class to get it to be reliable. (discussion thread)

Incorporeal creatures include Quells, Shadows, Spectres, Umbral Gargoyles, Umbral Worgs, Wheeps and Wraiths.

Players can gain an Incorporeal miss chance in some cases:
 * Ninja Spy Shadow Fade. 25% Incorporeal miss chance.
 * Shroud of the Wraith. 25% Incorporeal miss chance.
 * Ghostly. Currently 20% enhancement bonus to concealment. - Update 14 is going to change Ghostly to a 10% Incorporeal miss chance (stacking with Concealment, but no-longer stacking with other sources of Incorporeal miss chance).