Entropy
This is from The Elegant Universe:
[P]hysicists have given a fully quantitative definition to entropy [a measure of disorder or randomness] by using a definite numerical value: Larger numbers mean greater entropy, smaller numbers mean less entropy. Although the details are a little complicated, this number, roughly speaking, counts the possible rearrangements of the ingredients in a given physical system that leave its overall appearance intact. When your desk is neat and clean, almost any rearrangement […] will disturb its highly ordered organization. This accounts for low entropy. On the contrary, when your desk is a mess, numerous rearrangements of the [stuff] will leave it a mess and therefore will not disturb its overall appearance. This accounts for high entropy.