Entropy

Entropy is a measure of disorder in a system, and it always increases over time in closed systems.

This is because closed systems move towards a state of equilibrium, where energy is evenly spread out, and the amount of disorder increases as a result. This is described by the second law of thermodynamics, which states that the total entropy of an isolated system (one that doesn't exchange energy or matter with its surroundings) will always increase or stay the same, but it will never decrease.