Rgba stands for red, green, blue and what??

Question

i figure the code "rgba" is the abbreviation for Red, Blue, Green, and something starts with "a". Is "a" stands for "area"? My guess is that when the graphic is 3D, the value of "a" could then specify which color is for which surface.
Could someone help me understand this??

Answer 1

a is alpha which is for opacity. It ranges from 0.0 (completely transparent) to 1.0 (completely opaque)

Answer 2

Gotta love search engines...

https://www.w3schools.com/cssref/css_colors_legal.asp