A feature supported by some
image formats (such as
PNG but not
GIF) which is most easily described as "variable transparency". Whereas the GIF specification allows for binary transparency--pixels within the image may only be fully
opaque or fully
transparent--a PNG image can contain an "alpha channel", which assigns one of 254 (usually) levels of transparency to each pixel. When the rendering software, such as a web browser, draws a partially transparent portion of the image, it will allow the
colours "behind" the image to seep through to an extent which depends on the transparency level.
Alpha transparency is useful for creating visual effects which work well regardless of the background against which they are displayed. Common examples include drop shadows and anti-aliasing.