Unknown unknowns
知らないということを、知られていない事項のこと。

古くは1950年代に心理学者が提唱している概念だそうだけど、この言葉が有名になったのは今から20年前、同時多発テロ発生時のアメリカ国務長官ラムズフェルドの発言によって。

Reports that say something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknow