Complexity characterises the behaviour of a system or model whose
components interact in multiple ways and follow local rules, leading to non-
linearity, randomness, collective dynamics, hierarchy, and emergence.
The term is generally used to characterize something with many parts where
those parts interact with each other in multiple ways, culminating in a higher
order of emergence greater than the sum of its parts. The study of these
complex linkages at various scales is the main goal of complex systems theory.
The intuitive criterion of complexity can be formulated as follows: a system
would be more complex if more parts could be distinguished, and if more
connections between them existed.