What can 'a' be ?

Question

text-shadow: rgba(0,0,255,1) -1px -2px 0.5em; ....
r=red
g=green
b=blue
a=?

Answer 1

additional info:

RGBA color values are an extension of RGB color values with an alpha channel - which specifies the opacity of the object.

An RGBA color is specified with the rgba() function, which has the following syntax:

rgba(red, green, blue, alpha)

a is the opacity of the color (a = 0 = transparent; a = 1 = opaque).

Answer 2

a stands for alpha