The state of being impossible to understand; something that cannot be comprehended or deciphered.
彼の説明はあまりに専門的で、私には理解不能だった。
His explanation was so technical that I couldn't understand it at all.
その暗号は長年にわたって理解不能のままだった。
That cipher remained undecipherable for many years.
彼女の行動には理解不能なところが多い。
There are many aspects of her behavior that are incomprehensible.