The artist captured a completely serene expression on the subject's face.

serene 是形容词，意思是"平静的、宁静的"，用来形容一种安详、没有波澜的状态。这个词来自拉丁语 serenus，原指晴朗无云的天气，后来引申为形容人的表情或心境平和安宁。