Sentence:  <WSD>INVENTION</WSD>. A term used by J. S. Bach, and probably by him only, for small pianoforte pieces — 15 in 2 parts and 15 in 3 parts — each developing a single idea, and in some measure answering to the <span style="font-variant:small-caps">Impromptu</span> of a later day. 
Meaning of the word marked with <WSD> tags:  A small, self-contained composition, particularly those in J.S. Bach's "Two-" and "Three-part Inventions".