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Dictionary

pis•ton

Pronunciation: (pis'tun), [key]
—n.
1. 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.
2. a pumplike valve used to change the pitch in a cornet or the like.