Opacity

Opacity or opaque means something that is difficult to see through or perceive. It can also mean:

 Opacity (optics), the degree to which light is blocked.
 Opaque context, a linguistic context in which it is not possible to substitute co-referential terms while guaranteeing the preservation of truth values.
 Opaque data, in computer programming, data that can only be manipulated through an external interface.
 Opaque is a famous restaurant that serves customers in complete darkness.