Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it is often described as the degree of molecular disorder within a system. Entropy increases when the disorder or randomness increases, and decreases when the disorder decreases.
In closed systems, where there is no exchange of matter or energy with the surroundings, entropy tends to increase due to the natural tendency of particles to spread out and become more disordered over time. This is known as the second law of thermodynamics.
The concept of entropy applies to the universe as a whole, as it is a closed system. Over time, the universe tends towards a state of maximum entropy, where all energy is evenly distributed and no work can be extracted. This is known as the heat death of the universe.
In isolated systems, where there is no exchange of matter or energy with the surroundings, entropy can theoretically decrease. However, this would require a highly improbable and carefully controlled scenario, as the natural tendency is for entropy to increase.
The concept of entropy is closely related to the arrow of time, as entropy tends to increase in one direction of time. This is because the increase in entropy is a measurable and observable phenomenon, while the reverse (decrease in entropy) is not. This is known as the thermodynamic arrow of time.