the meaning of 'impromptu'

[allchopin]

Does impormptu mean  that the composer made up the song on the fly (like un public) or is it simply a type of piece with a  certain style and has nothing to do with imporvisation?

[BuyBuy]

"Impromptu" is a French word that describes something done at the last minute, non planned.

When used as a title of a piece, it usually doesn't mean it's been composed as an improvisation, but it rather stands for a piece whose form is free and flowing, as if it was improvised.

ex : Chopin's impromptus, Fauré's impromptus, Listz's valse-impromptu...

[chopiszte]

BuyBuy said it perfectly.  Impromptus are often very impressive sounding due to the fact that, ostensibly, they are spontaneous.  

Thus, Impromptus can be musically demanding.  They must sound as if they were improvisations.

I am sure many impromptus were results of improvisation. It is unfortunate Beethoven or Mozart, the greatest improvisers ever, did not compose impromptus frequently like schubert and Chopin.  Was the style even existent then?

[BuyBuy]

Well, did you know that quite a few of Mozart's sonatas were composed on the go ? He used to improvise them in concerts, until he decided to write down some of them.