Nested
Structure
An outer propagator correction (e.g. a photon arc spanning the full diagram) encloses an inner propagator correction (a shorter arc between interior vertices). The inner loop is contained within the outer loop.
Where it appears
Nested rainbow in the two-loop electron self-energy: outer photon from $v_1$ to $v_4$, inner photon from $v_2$ to $v_3$.