Alpha blending

From GMpedia.org Wiki

The term Alpha blending is a value referring to how transparent is an image. An image's alpha blending is a numerical value, usually an integer from 0 to 255, but is sometimes a decimal, ranging from 0 to 1. The least value is fully-transparent and the greatest value is fully-opaque.

It is similar to an alpha channel except for the alpha blending being for the entire image.