An overly simple definition of complexity according to systems theory

Problem spaces can be simple, complex and chaotic.

Simple

you can clearly identify actor, input and output

Complex

There are too many factors in play. Complex systems rely a lot on situational data for their outcome and are rarely (if ever reproducible). That said, reason out the output afterwards. An example of a complex system are elections.

One can navigate these system by tweaking aspects of them and seeing how it plays out (In many ways, it sounds a lot like machine learning).

Chaotic

Impossible to predict or understand an outcome. An eldritch horror of sorts...

Disorder is not knowing how to fit a problem space into one of the aforementioned categories.