Piston

A piston is a moving component that fits within a cylinder and transmits force between a fluid pressure and a mechanical linkage. For an effective face area $A$ and pressure difference $\Delta p$, the ideal axial pressure force is $F=A\Delta p$. Sliding seals reduce leakage but introduce friction, and connecting rods can add side loads. In an engine the piston converts pressure work into motion; in a compressor mechanical work instead raises fluid pressure. The swept volume is face area times stroke for a constant-bore cylinder.