Geometrical frustration
Geometrical frustration is a phenomenon in condensed matter physics in which the geometrical properties of the atomic lattice forbid the existence of a unique ground state, resulting in a nonzero residual entropy. The most important consequence of this is that the entropy of the system does not go to zero at absolute zero. An example of a geometrically frustrated material is ordinary water ice, which has a residual entropy of around 3.40 J/K/mol.