形容词

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.

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