piston
/ˈpɪstən; `pɪstn/ n
1 round plate or short cylinder,usu made of metal or wood, that fits closely inside another cylinder or tube and moves up and down or backwards and forwards inside it; used eg in steam or internal combustion engines to cause other parts to move by means of a connecting rod (`piston-rod) 活塞. =>illus 见插图.
2 sliding valve in a trumpet or other brass wind instrument （铜管乐器上的）活门.