Ethereal

Ethereal (sometimes called Ghost or Decrep) is a type of status effect that slows and makes the target unable to attack. Afflicted units can still cast spells, but also take additional damage from spells.

Ethereal abilities are excellent counters to heroes which rely on their auto-attack to do damage, such as Ursa and Anti-Mage. It can also be used in order to maximize spell damage against specific targets.