[gilleain]

Mostly overlapping definition of what a 'complex system' is with :

https://en.wikipedia.org/wiki/Complex_system

although I understood the key part of a system being complex (as opposed to complicated) is having a large number of types of interaction. So a system with a large number of parts is not enough, those parts have to interact in a number of different ways for the system to exhibit emergent effects.

Something like that. I remember reading a lot of books about this kind of thing a while ago :)