Disorder in a system that tends to increase over time until reaching an equilibrium. Factors of entropy include heat, time, and the number of things in the system.
Entropy is measured by the number of macro states (e.g. arrangement of molecules) the system can be in where a value of 1 would be a single state (i.e. crystal structure).
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User experience for a non-trivial web UI tends to degrade over time as more functionality is added and more engineers add to it.