• Opacity (optics)
  Opacity (optics)
  Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
  
  , the degree to which light is not allowed to travel through
• Opaque context
  Opaque context
  An opaque context is a linguistic context in which it is not always possible to substitute co-referential expressions salva veritate. In other words, substitution of co-referential expressions into an opaque context does not always preserve truth...
  
  , a term to describe the linguistic context of co-referential terms
• Phonological opacity
  Phonological opacity
  Phonological opacity was first defined by Kiparsky as a measure of how far the context or the consequences of a phonological process may be determined only by examining the surface structure...
  
  , a term in phonology
• Opaque travel inventory
  Opaque travel inventory
  Opaque inventory is a term used to describe the market of selling unsold travel inventory at a discounted price. The inventory is called "opaque" because the specific supplier remain hidden until after the purchase has been completed...
  
  , the market of selling unsold travel inventory at a discounted price

Computer science

• How much an image will obscure the background when painted, usually defined by its alpha channel
  Alpha compositing
  In computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial or full transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a...
  
  
• The use of an opaque data type
  Opaque data type
  In computer science, an opaque data type is a user defined data type used like built-in data type. It is incompletely defined in an interface, so that ordinary client programs can only manipulate data of that type by calling procedures that have access to the missing information.-Overview:Opaque...
  
  , a data type that is incompletely described in a module interface
• Said of data that can only be interpreted and by an external entity, such as a magic cookie
  Magic cookie
  A magic cookie or just cookie for short, is a token or short packet of data passed between communicating programs, where the data is typically not meaningful to the recipient program. The contents are opaque and not usually interpreted until the recipient passes the cookie data back to the sender...
  
  
• Opaque binary blob
  Opaque binary blob
  Opaque binary blob is a term used in network engineering and computer science to refer to a sizeable piece of data, which looks like binary garbage from outside, by entities which do not know what that blob denotes or carries, but make sense to entities which have access permission and access...
  
  , a term used in network engineering and computer science