Disorder is measured in terms of a quantity called entropy. The more disordered a system is, the higher its entropy. So another way of expressing the second law is to say that entropy always increases, or at best stays the same. In other words, the future towards which the arrow of time points is the direction in which entropy is greater. There is more order in a more structured system, such as [water plus ice], than there is in a more uniform system, such as [water], because it takes more information to describe an ordered system.
