What is "fantaisie-impromptu"

Frédéric Chopin's Fantaisie-Impromptu in C-sharp minor, Op. posth. 66, is a solo piano composition. It was composed in 1834 and published posthumously in 1855 although he had instructed that none of his unpublished manuscripts should be published. Despite negative comments, the Fantaisie-Impromptu is one of Chopin's most frequently performed and popular compositions.