Definitions of complexity often depend on the concept of a "system" – a set of parts or elements that have relationships among them differentiated from relationships with other elements outside the relational regime.
Definition of complexity noun from the Oxford Advanced Learner's Dictionary. [uncountable] the state of being formed of many parts; the state of being difficult to understand. I was astonished by the size and complexity of the problem. Only now did he understand the full complexity of the problem.
He was impressed by the complexity of the music. The diagram illustrates the complexity of the cell's structure. He doesn't grasp/understand the complexity of the situation.
The complexities of something are the many complicated factors involved in it. ...those who find it hardest to cope with the complexities of modern life.
He was impressed by the complexity of the music. The diagram illustrates the complexity of the cell's structure. He doesn't grasp/understand the complexity of the situation.