In optics, transparency is the property of being transparent, i.e. allowing light to pass. For example air and some other gases, clear water, various other liquids, glass (usually), and plastic (sometimes, e.g. perspex), are transparent. If the amount of transparency depends on the wavelength of the light then the glass, etc., is tinted. This may for instance be due to certain metallic oxide molecules in glass, or (larger) colored particles, as in (light) colored smoke. If many of these colored particles are present the gas, liquid or solid becomes opaque, e.g. dense smoke.
There are transparent glass walls that can be made opaque by the press of a button, see ICE.
"Transparent" materials can be seen through (such as a common glass window), whereas "translucent" materials allow some light to pass through them but cannot be seen through (for example frosted glass).