Opacity

Opacity is the opposite of transparency. It is a measure of how much light passes through the paper.
The greater the opacity of the paper, the less light comes through, and the lower the opacity of the paper, the more light penetrates.

For the production of a book, a paper with high opacity is recommended (the paper is then opaque!), especially for two-sided printing, so that the printed image on the back will not show through to the front.