Alpha

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This sign from Half-Life 2 is not a model, it's a texture that uses an alpha channel for transparency.
For the VMT parameter, see $alpha.

Alpha is a byte variable that is often included as the "fourth" byte in a RGB color format. Alpha is usually an opacity rating from 0 - 255, where 0 is completely invisible and 255 is completely visible. Alpha is sometimes also used for other things, such as a self-illumination mask.

To store an individual alpha value in each pixel, an alpha channel is used as the fourth channel of the TGA file. This allows variable opacity, for example, in a single texture, such as the lettered Petrol signs in Half-Life 2.

Paint.net is a good program for adding alpha to a texture. Be aware that a pixel with alpha set to 0 still has R, G, and B values, which may affect the texture in some cases.