A lot of comments mention John Ousterhout's book Philosophy of software design and it's definition of complexity of a system being cognitive load (I.e the number of disparate things one has to keep in mind when making a change). However IIRC from the book, complexity of a system = Cognitive load * Frequency of change.

The second component, frequency of change is equally important as when faced with tradeoffs, we can push high cognitive load to components edited less frequently (eg: lower down the stack) in exchange for lower cognitive load in the most frequently edited components.