translucency - technical definition

The quality of being able to see through a material whereby the distant image is hazy or foggy. The terms "translucent" and "transparent" are often used synonymously, but they are not the same. A translucent area in an image would be like looking through frosted or smoked glass to the underlying background. A transparent area would be like looking through clear glass.

Not Well Supported
True translucency is not supported in many graphics formats as an attribute of the image. Although the visual effect of translucency can always be simulated in an image by using an image editing program, the actual translucent area is opaque, and the underlying background does not show through.

Translucency is an advanced technique; for example, Adobe's PostScript and HP's PCL language do not support translucency; however, Adobe's PDF, which came later, does support it. Images formatted with true translucency that are printed in PostScript and PCL are flattened into opaque images that tend to have visual artifacts. See alpha blending.