$alpha
From Valve Developer Community
Scales the opacity of an entire material by the given value. 1 is entirely opaque, 0 is invisible.
If any material on a brush has $alpha, the brush will stop affecting VIS and become entirely transparent to NPCs from every angle.
This may be used with $translucent.
Syntax
$alpha <normal>
See also
$translucent, for per-pixel opacity.
