<p><strong>Noun</strong></p><ul><li>A disk or cylindrical part tightly fitting and moving within a cylinder, either to compress or move a fluid collected in the cylinder, as air or water, or to transform energy imparted by a fluid entering or expanding inside the cylinder, as compressed air, explosive gases, or steam, into a rectilinear motion usually transformed into rotary motion by means of a connecting rod.</li><li>A pumplike valve used to change the pitch in a cornet or the like.</li></ul><p><strong>End of Response</strong></p>