A tonometer measures the pressure inside your eye, which is called Intraocular Pressure (IOP). This test is used to check for glaucoma, an eye disease that can cause blindness by damaging the nerve in the back of the eye (optic nerve). 'Machmeter' is an instrument in an aircraft indicating airspeed as a Mach number. 'Viscometer' is an instrument used to measure the viscosity of a liquid. 'Odometer' is an instrument for measuring the distance travelled by a wheeled vehicle.