ETYM Latin opacitas: cf.French opacité.
Quality or state of being opaque. opacify, v. make or become opaque.
1. Incomprehensibility resulting from obscurity of meaning; SYN. opaqueness.
2. Not permitting the passage of light; SYN. opaqueness.
3. The quality of being opaque to a degree; the degree to which something reduces the passage of light; SYN. opaqueness.

The quality that defines how much light passes through an object’s pixels. If an object is 100 percent opaque, no light passes through it.