Tonometry: A standard eye test that is done to determine the fluid pressure inside the eye. Increased pressure is a possible sign of glaucoma, a common and potentially very serious problem if not detected and treated promptly. The pressure inside the eye is measured from the outside. In most cases, the pressure can be measured without anything actually touching the eye. The patient looks at an instrument that blows a small puff of air into the eye and then uses a special kind of sensor to detect the amount of indentation that the air puff causes on the surface of the eye. This indentation is normal and lasts only a fraction of a second. If patients need to have their eye pressure measured when this type of machine is not available (as in an emergency room), the pressure can be measured with an instrument that resembles a pen. One end of the instrument is placed on the surface of the eyeball. This feels like having a contact lens put in the eye. Tonometry does not cause significant pain and it is risk free.