a performance that an actor, musician, etc. has not practised or planned

a blues/jazz improvisation 即兴演奏的布鲁斯／爵士乐

There are classes in movement, dance, and improvisation. 有音乐、舞蹈和即兴表演方面的课程。

the act of making or doing something with whatever is available at the time

I'm afraid we don't have all the necessary equipment, so a little improvisation might be required. 恐怕我们的必需设备不全，所以我们可能需要把东西临时凑合一下。