Transmission density is a term used to describe the level of light that is able to pass through a material. Synonyms for transmission density might include terms such as opacity, transparency, or translucency. These terms refer to the degree to which light is able to pass through an object. Another related term might be "refractive index," which describes the degree to which light is bent or refracted as it passes through an object. Additionally, terms such as "light scattering," "light absorption," and "light transmission" may also be used to describe the behavior of light as it passes through an object, and can be considered synonyms for transmission density.