transformed (vec3)

Vertex position in model space with skinning applied.

// begin_vertex.glsl.js

vec3 transformed = vec3( position );

It's adjusted in skinning_vertex.glsl.js if USE_SKINNING is defined, but in most cases this is synonymous with vertex position.

mvPosition (vec4)

Vertex position in camera space. Remember that +Z points out from the screen, so all the stuff in the camera's view will have a negative Z value for this vector.

This can also be interpreted as a vector pointing from the camera to the vertex in camera space

// project_vertex.glsl.js

vec4 mvPosition = vec4( transformed, 1.0 );
mvPosition = modelViewMatrix * mvPosition;

vViewPosition (vec3)

Negated XYZ components of mvPosition. You should interpret this as the camera position in "vertex space" (I don't know if that's a real term — I mean the camera position relative to an origin at the vertex).

This can also be interpreted as a vector pointing from the vertex to the camera in camera space.

// meshphysical_vert.glsl.js

vViewPosition = - mvPosition.xyz;

Here's a shitty sketch attempting to summarize these variables in two dimensions:

viewDir (vec3)

Normalized version of vViewPosition . This is a unit vector pointing from the vertex to the camera in camera space.

// meshmatcap_frag.glsl.js

vec3 viewDir = normalize( vViewPosition );