[pis.ton] n [F, fr. It pistone, fr. pistare to pound, fr. OIt, fr. ML, fr. L pistus, pp. of pinsere to crush--more at pestle] (1704) 1: a sliding piece moved by or moving against fluid pressure which usu. consists of a short cylinder fitting within a cylindrical vessel along which it moves back and forth
2. a: a sliding valve moving in a cylinder in a brass instrument which when depressed by a finger knob serves to lower the instrument's pitch b: a button on an organ console to bring in a previously selected registration