Syringe A simple hydraulic machine, employed to draw in and discharge fluids violently. It consists merely of a small tube, in which is fitted a piston or plunger, and having a small hole at the bottom of the tube, through which the liquid enters, when the plunger is drawn back, and charges the barrel. Then, by forcing the plunger forwards, the fluid is expelled with a violence proportioned to the velocity given to the plunger.