即席的英文：
extempore
impromptu
off the cuff
off the top of one's head
on-the-spot

参考例句：

A speech made impromptu.
即席演说。

He made an impromptu speech.
他做了即席演说。

The pianist improvised an accompaniment to the song
那位钢琴家为那首歌即席伴奏。

The speaker can extemporize on any of a number of subjects.
该演讲人对若干题目中的任一个都能即席发言。

He had to extemporize because he had forgotten to bring his notes.
他因为忘了带讲稿,只好即席发言.

I spoke for an hour easily, and from that time always extemporized.
我轻而易举地讲了一个钟头，从那以后我就总是即席讲话了。

bravely demonstrating his ability to extemporize intelligently(William Safire)
勇敢地展示他的智慧的即席表演才能(威廉·萨菲尔)

Fake an accompaniment
即席伴奏

An accomplished extemporaneous speaker.
一位高明的即席演讲者

I am ready to give the speech off the top of my head.
我准备好即席发言了。

extempore是什么意思：
adj. 即席的，当场的
adv. 即席，当场
n. 即席之作

The speaker can extemporize on any of a number of subjects.
该演讲人对若干题目中的任一个都能即席发言。

bravely demonstrating his ability to extemporize intelligently(bWilliam Safire)
勇敢地展示他的智慧的即席表演才能(b威廉·萨菲尔)

He have to extemporize because he have forget to bring his note.
他因为忘了带讲稿，只好即席发言。

impromptu是什么意思：
n. 即席演出，即兴曲
adj. 即席的

There are eight such impromptus in total.
总共有8个这样的即兴曲。

A piece of impromptu spiced up the concert.
一段即兴曲使音乐会更活跃了。

The children put on an impromptu concert for the visitors.
孩子们为来访者即兴献上了一场音乐会。

off是什么意思：
adv. 掉，下；离开；隔开；切断；停止；；休息；光，完，尽；全部地
prep. 从...离开；从...向下；在...的外面；偏离...；从...分岔；不再从事...；低于...
adj. 关着的；停止流通的；延期的；取消的；不工作的；处于...境况的；萧条的
v.（用于祈使句）走开，滚开；站开
n. 关掉

Is the water on or off?
自来水龙头开着还是关着？

The gilt is off.
幻想破灭了。

to nip off; to cut off; to break
掐断