形容词

1. The eruption of a volcano is spontaneous.
火山爆发是自发的。

2. Hearing the joke, we burst into spontaneous laughter.
听到笑话，我们不由自主地大笑起来。

3. He made a spontaneous offer of help.
他自愿提供帮助。

1. (行为)自然的，自发的
Spontaneous acts are not planned or arranged, but are done because someone suddenly wants to do them.

e.g. Diana's house was crowded with happy people whose spontaneous outbursts of song were accompanied by lively music...
戴安娜的家里挤满了兴奋的人们，他们伴着欢快的音乐不由自主地引吭高歌。
e.g. I joined in the spontaneous applause.
我跟着也自然而然地鼓起掌来。

adj

1. said or done without having been planned or written in advance

e.g. he made a few ad-lib remarks

Synonym: ad-libunwritten

2. happening or arising without apparent external cause

e.g. spontaneous laughter
spontaneous combustion
a spontaneous abortion

Synonym: self-generated