surprise examination; pop quiz
Refers to an unexpected test or exam, often given without prior notice.
We had a surprise exam today and I couldn't prepare at all.
抜き打ちテスト is a more common and colloquial term for a surprise test, while 不意試験 is a straightforward compound that is less frequently used.
Compound of 不意 (sudden, unexpected) and 試験 (examination). The exact origin is uncertain, but it follows a standard Sino-Japanese word formation pattern.