View space
The world re-expressed so the camera sits at the origin looking down a fixed axis — the space where "how far away" collapses into a single coordinate.
The camera never actually moves. The view matrix, the inverse of the camera's own transform, shifts and rotates the entire world the opposite way until the camera lands at the origin. By OpenGL convention it then looks down negative Z; in Direct3D, positive Z — a sign flip that has eaten more afternoons than anyone cares to admit.
The space is handy because distance from the viewer becomes trivial to read off. Fog, depth sorting of transparent surfaces and plenty of screen-space effects such as SSAO do their maths here, and deferred renderers long stored their normals in view space. The next step, the projection matrix, sends the geometry on to clip space.