What is a complex system?
A complex system is one composed of parts that as a whole exhibit properties or behaviours not obvious from the properties of the individual parts. Organisms are complex systems made up of organs, which are themselves complex systems made up of cells made up of complex systems of molecular parts.
A mere catalog of parts cannot predict the behaviour of complex systems.
As scientists, we would like to understand how the dynamics of these systems emerge from the sum of their parts.