alpha blending 
In computer graphics, each pixel has three channels of color information--red, green, and blue--and sometimes a fourth, called the alpha channel. This channel controls the way in which other graphics information is displayed, such as levels of transparency or opacity. Alpha blending is the name for this type of control. It's used to simulate effects such as placing a piece of glass in front of an object so that the object is completely visible behind the glass, unviewable, or something in between.