Final answer:
On the piano, the black key between the white keys C and D is called C-sharp or D-flat, depending on the musical context.
Step-by-step explanation:
The black key between the white keys C and D on the piano is called C-sharp or D-flat. The nomenclature depends on the musical context, such as the key or the scale in use. In a C major scale, we would refer to this black key as C-sharp, while in a D-flat major scale, we would call it D-flat. This black key can be used to play notes in both scales, and its dual name reflects its versatile function in Western music.