melopiano

from The Century Dictionary.

• noun A form of pianoforte, invented by Caldara in 1870, on which a sustained tone, with a chance for crescendo and diminuendo effects, is made possible through an ingenious arrangement of little hammers that strike rapidly upon the strings and thus prolong and control their vibration. The quality of the tone produced is sweet and effective.

from the GNU version of the Collaborative International Dictionary of English.

• noun A piano having a mechanical attachment which enables the player to prolong the notes at will.