There are several cards, such as Arcane Nullifier, Spectral Knight, and Faerie Dragon that have the card text "cannot be targeted by spells or hero power". I haven't been able to find an "official" term for this ability online. Is there an official or unofficially accepted name?
There is none as of yet. There's been debate in the HS community about several options. Veil being my personal favorite, but nothing mentioned by Blizzard as of yet. If there was an official name, it would be on the cards, no?
Here is some discussion on it.