Dr. Richard Spaulding discuss relattionships and how complex they can be. Complexity characterises the behaviour of a system or model whose components interact in multiple ways and follow local rules, meaning there is no reasonable higher instruction to define the various possible interactions.
The stem of the word "complexity" - complex - combines the Latin roots com (meaning "together") and plex (meaning "woven"). Contrast "complicated" where plic (meaning "folded") refers to many layers. A complex system is thereby characterised by its inter-dependencies, whereas a complicated system is characterised by its layers.
Complexity 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. Just as there is no absolute definition of "intelligence", there is no absolute definition of "complexity"; the only consensus among researchers is that there is no agreement about the specific definition of complexity. However, "a characterization of what is complex is possible".