A physical system is frustrated if it has no well-defined ground state because there are competing interactions among the pieces of the system that cannot simultaneously be at an energy minimum. A simple example is a system of three spins. If the interaction energy between two spins is lowest when they point in opposite directions, the ground state of a pair of spins is clearly for the two spins to point in opposite directions. If a third spin is added, it is pulled in opposite directions attempting to minimize its interaction with the other two.