The alpha byte

The value of the alpha byte can range from 0 to 255.

When rendered using a mechanism that supports alpha transparency,

• an alpha value of zero
• causes the pixel to be totally transparent.

Similarly, an alpha value of 255

• causes the pixel to be totally opaque.

Values between zero and 255

• cause the pixel to be rendered as partially opaque or partially transparent,
• whichever terminology you prefer.