to invent or make something, such as a speech or a device, at the time when it is needed without already having planned it

I hadn't prepared a speech so I suddenly had to improvise. 我没有准备演讲，所以突然之间我只得即兴讲几句。

We improvised a mattress from a pile of blankets. 为了能有地方睡觉，我们临时用一摞毯子作床垫。

When actors or musicians improvise, they perform without prepared speech or music, making up the play, music, etc. as they perform it.

During certain scenes of the play there isn't any script and the actors just improvise (the dialogue). 这出戏有几场没有剧本，演员们就即兴表演（对白）。