A time crystal is a type of physical system that exhibits a type of temporal or time-based periodicity that cannot be explained by the conventional laws of thermodynamics. It is a form of matter that repeats its structure over time in a non-equilibrium state, even at absolute zero temperatures. Time crystals are related to the concept of discrete time crystals, which are structures that show a repetitive behavior over a period of time, but do not require temperature to maintain the structure.