Something sudden or unexpected, occurring without warning or preparation (often used as '不意に').
不意に電話が鳴って驚いた。
I was surprised when the phone rang unexpectedly.
彼の不意の質問にうまく答えられなかった。
I couldn't answer his unexpected question well.
不意を突かれて相手は言葉を失った。
Caught off guard, the other person was at a loss for words.
彼女は不意に姿を消した。
She disappeared suddenly/without warning.