What is a frustrated system?

Frustration is typically the result of having a highly degenerate ground state. Consider a system of 3 spin-1/2 particles on the vertices of a triangle, with a pairwise anti-ferromagnetic interaction. This system has a triply degenerate ground state (1u 2u 3d or 1u 2d 3u or 1d 2u 3u) and is thus said to be frustrated. The concept can be extended from there to a triangular lattice, or a cubic lattice with different interactions and further to disordered, amorphous materials. A spin glass is a frustrated spin system that exhibits the weird, long-time dynamics of glassy systems.