tl;dr : What is Infrastructure Entropy?

The Law of Software Entropy as described by Ivar Jacobson et al. in "Object-Oriented Software Engineering: A Use Case Driven Approach":

The second law of thermodynamics, in principle, states that a closed system's disorder cannot be reduced, it can only remain unchanged or increased. A measure of this disorder is entropy. This law also seems plausible for software systems; as a system is modified, its disorder, or entropy, always increases. This is known as software entropy.

Infrastructure suffers from the same issues around change. Configurations, data tables, patches and hotfixes all contribute to infrastructure entropy.

The greater the entropy, the more fragile the system becomes and the more likely it is the next change will bring it down. This is a condition we (obviously) want to avoid.