Overlapping
Two loop structures that partially overlap — neither one encloses the other. The defining feature of irreducibly two-loop diagrams that cannot be decomposed into nested one-loop subdiagrams.
Structure
Two propagator arcs that cross: one spans vertices $(v_1, v_3)$ while the other spans $(v_2, v_4)$, with vertex ordering $v_1, v_2, v_3, v_4$ along the fermion line. Neither arc contains the other.
Where it appears
Overlapping rainbow in the two-loop electron self-energy. This topology is the hardest to compute because it cannot be reduced to products of one-loop integrals.