an electric railway operating below the surface of the ground (usually in a city); "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
the eye unaided by any optical instrument that alters the power of vision or alters the apparent size or distance of objects; "it is not safe to look directly at the sun with the naked eye"
increased pressure in the eyeball due to obstruction of the outflow of aqueous humor; damages the optic disc and impairs vision (sometimes progressing to blindness)