A material presents transparency when it allows light to pass through easily. Transparency is an optical property of matter, which has various degrees and properties. A material is translucent when it allows light to pass through, but does not allow objects to be seen clearly. On the other hand, it is opaque when it prevents the passage of light. Other categories of visual appearance that are related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesía "Cesía (visual appearance)") in an ordering system with three variables, which include transparency, translucency and opacity among the aspects involved.

Generally, a material is said to be transparent when it is transparent to visible light. For technical applications, transparency or opacity to infrared radiation, ultraviolet light, X-rays, gamma rays or other types of radiation is studied.

According to quantum mechanics, a material will be transparent at a certain wavelength when in its energy level scheme there is no energy difference that corresponds to that wavelength. Thus, air and glass are transparent, because in their energy level schemes (or energy bands, respectively) there is no room for any difference in energy on the order of visible light. However, they can absorb, for example, part of the infrared radiation (water and carbon dioxide molecules absorb in the infrared) or ultraviolet (glass blocks part of the ultraviolet spectrum).

Transparency is quantified as transmittance, the percentage of light intensity that passes through the sample. For this, a colorimeter or a spectrophotometer is used.