an occasion when people wrongly believe that something dangerous or unpleasant is happening or will happen

Three fire engines rushed to the school only to discover it was a false alarm. 3辆救火车疾驰到学校，结果却发现是假火警。

She thought she was pregnant, but it turned out to be a false alarm (= she was not). 她以为自己怀孕了，但后来发现是一场虚惊。