Complexity theory and related concepts emerged quite recently, towards the late 20th century. Complex means diversity, multiple concurrent interdependencies between different elements of a system. Human cells in itself are a very complex system. However they can self-organize and form groups. Create variations and give rise to even more complex systems like a human.
Complexity theory studies complexity systems. A system composed of many components that interact and have different dependencies with each other. Key concepts include systems, complexity, networks, nonlinearity, emergence, self-organization, adaptive.
A business is also a highly-complex system. Comprising of complex individuals both in an around the business interacting with it simply by creating products, selling/buying them.