noun which may take the genitive case particle 'no', noun
unprecedented; unheard-of; record-breaking
Describes an event or situation that has never happened before, often with a dramatic or serious tone. Commonly used in news and formal contexts.
未曾有の災害に、国中が衝撃を受けた。
The whole country was shocked by the unprecedented disaster.
未曾有の大雨で川が氾
The river flooded due to record-breaking heavy rain.
空前 also means 'unprecedented' but is more neutral and often used in phrases like 空前絶後 (unprecedented and never to be repeated). 未曾有 carries a stronger nuance of calamity or extraordinary scale.
前代未聞 is a more emphatic, idiomatic expression meaning 'unheard-of in previous generations', often used for shocking or scandalous events.
From classical Chinese 未曾有 (wèicéngyǒu), literally 'not yet ever existed'. The Japanese reading みぞう is a kan'on reading of the characters.