Delta–wye transformation
also known as: Star-delta · Y-Δ
The equivalence between three resistances arranged in a triangle (delta) and three in a star (wye): any delta has an exactly equivalent wye and vice versa. For balanced networks, Z_Y = Z_Δ/3.
In practice
It un-sticks network analysis when nothing is purely series or parallel (bridge circuits), and it's physical reality in three-phase machines — a motor started in wye sees 1/3 the per-winding power, which is the classic soft-start trick.