The first recorded use of the spouse impromptu in this sense occurred in 1817, in the Allgemeine musikalische Zeitung, an idea of the publisher to describe a piano piece by Voříšek.
It didn't take long before the first generation of Romantic composers took up the idea:
The impromptu genre remained popular all throughout the 19th century.
In the 20th century there are fewer examples of composers naming their compositions "Impromptu", e.g.: