She learned an important lesson from failing that exam, namely that nothing is ever certain.

namely 是副词，意思是"即、也就是"，用于引出具体说明或举例。这个词来自古英语，由 name（名字）和 -ly（副词后缀）组成，字面意思是"指名道姓地"，引申为"具体说明"。