In optics, the extent to which an object or surface will impede the transmission of light through it. A completelyopaque object is one which allows no light to pass through it. See also Translucency and Transparency.
In papermaking, opacity is a property of paper that describes the amount of light which is transmitted through it. Paper that has a high degree of opacity does not let much light pass through it, while paper that has a low degree of opacity is more translucent, or allows much light to pass through it. A paper’s opacity determines the extent to which printing on a particular side of paper will be visible from the reverse side (called show-through).