Webster's English Dictionary

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piston

pis.ton \'pis-t*n\ n [F, fr. It pistone, fr. pistare to pound, fr. ML 
   pistare, fr.] L pistus, pp. of pinsere to crush - more at PESTLE 1: a 
   sliding piece moved by or moving against fluid pressure that usu. cons ists 
   of a short cylinder fitting within a cylindrical vessel along which it 
   moves back and forth 2a: a valve sliding in a cylinder in a brass wind 
   instrument and serving wh en depressed by a finger knob to lower its pitch 
   2b: a button on an organ console to bring in a previously selected registra 
   tion