| A technique used to render a graphic object transparent or translucent. At it's simplest, it's just a modification in color based on a specific attribute of an object.
As well as having an RGB value to determine a color, each pixel has an alpha channel which determines it's transparency.
Each pixel in a texture rendered onto an object is assigned an alpha value. This value is combined with the alpha values of objects logically behind it view to gain the final image.
The lower the alpha value, the more transparent the object becomes. In other words, an object with a low alpha value will not modify the appearence of the object behind it by very much.
For example, let's imagine we're trying to create the effect of looking through a stained glass window. In this case, each object "behind" the glass (Remeber, a 3D effect is only an illusion) would be combined with the glass it in such a way that the obects when rendered are modified slightly in color depending on the alpha value of the glass object. The higher the value, the greater the object will be tinted with this color.
This effect can of course be cumulative. | 
