spontaneous

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

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。

2. 自然的，自发的(指由于内部原因而非外界影响所引起的)
A spontaneous event happens because of processes within something rather than being caused by things outside it.

e.g. I had another spontaneous miscarriage at around the 16th to 18th week.
我在大约第 16 至 18 孕周时又经历了一次自然流产。
e.g. ...a spontaneous explosion.
自爆