An open system in non-equilibrium with its environment that exhibits time translation symmetry breaking.

While the usage of the term is still debated, the general idea is that time crystals (which have been made using ytterbium ions and defect-bearing diamonds) break temporal symmetry analogously to how mineral crystal break spatial symmetry. While spatial crystals are inhomogeneous in space (vs a gas, which is distributed evenly) due to the ordering of their atoms, the properties of atoms in a time crystal are inhomogeneous in the time dimension.