Template:Shaderparam It defines an albedo texture. It is very rare for a material not to use this parameter and some shaders will load a default texture if it stays undefined.

VMT syntax

$basetexture	<texture without extension, relative to <game>\materials\>

LightmappedGeneric
{
	$basetexture	brick\brickwall031b
}

Additional parameters

Some shaders (most notably WorldVertexTransition) allow the use of two albedos. The second texture is handled by a mirrored set of commands with '2' in their name: $basetexture2 $basetexturetransform2 $frame2

There are further '2' commands applying to various shader effects, which are each documented in their respective articles.