In response to an unforeseen need; SYN. spur-of-the-moment.
Extempore (composition).

1. A short musical passage that seems to have been made spontaneously without advance preparation.
2. An extemporaneous speech or remark;
In music, a 19th-century character piece in the style of an improvisation. Composers of piano impromptus include Schubert and Chopin.