a.

1. 自发的；非出于强制的
   He made a spontaneous offer of help.

   他自愿提供帮助。

2. （动作等）无意识的，不由自主的
   Hearing the joke, we burst into spontaneous laughter.

   听到笑话，我们不由自主地大笑起来。

3. 自然的
   spontaneous behavior

   自然的举止

4. （植物）自生的，非人工栽培的
5. （文体等）自然而优美的

adj.

1. performed or occurring as a result of an unpremeditated inner impulse and without external stimulus.
   ▸(of a process or event) occurring without apparent external cause.
   ▸Biology  (of movement or activity) instinctive or involuntary.
2. open, natural, and uninhibited.
3. archaic (of a plant) growing naturally and without being tended or cultivated.