nitrous oxide
noun [ U ] chemistry specialized uk/ˌnaɪ.trəs ˈɒk.saɪd/ us/ˌnaɪ.trəs ˈɑːk.saɪd/ (informal laughing gas)
一氧化二氮;笑气（一种带甜味的气体，有轻微的麻醉作用，常用于牙科止疼）
a type of gas with a sweet smell that is used as a weak anaesthetic (= substance that stops pain), especially by dentists