Nested
One loop structure completely enclosed inside another. The inner subdiagram sits between two vertices of the outer one. Appears at two loops and above.
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 $v_3$.