Impromptu

An impromptu is a free-form musical composition with the character of an ex tempore improvisation as if prompted by the spirit of the moment, usually for a solo instrument, such as piano. According to Allgemeine musikalische Zeitung, Johann Baptist Cramer began publishing piano pieces under the (sub-)title of "impromptu.", which seems to be the first recorded use of the term impromptu in this sens... (Wikipedia)