As an adjective, this shape names a square lattice model developed by Kitaev with a spin-1/2 (15[1])particle on each edge, which exhibits abelian anyons and is fault-tolerant due to its topological order. Certain perturbed quasiperiodic orbits will remain on an "invariant" one of these shapes according to the K·A·M theorem. A 2D crystal's first Brillouin zone has the topology of this shape, since it is a square with periodic boundary conditions. (15[1])The poloidal component points along the shorter way to (*) wind around one of these surfaces. Sakharov (10[1])and Tamm showed that twisting a magnetic field along a chamber of this shape could confer enough stability to reach temperatures needed for ignition. Plasma is (10[1])confined into one of these shapes in a fusion reactor called a (10[1])tokamak. For 10 points, name this shape that resembles a donut. ■END■