The density of a color or the density of a piece of paper.
The opacity of an image or object can range from transparent (0% opacity) to opaque (100% opacity). The measure of opaqueness or how much you can see through a graphic or a piece of paper is its opacity. The more fibers or fillers in a paper, the more opaque (less see-through) it will be. Thinner papers can be made more opaque with opacifying thickeners.
use it in real life: Let's say you want to print a very saturated color on one side of a page, and on the other you have lots of white space. You will want to choose a paper with a good amount of opacity to ensure you don't see some of that intense color bleeding through to the white side of the paper.