Complexity theory is a subdivision of theoretical computer science that deals with the difficulty of solving computational problems. It might not be immediately apparent, but complexity theory (and its many facets) has a relationship to SEO.
For our purposes, it applies, conceptually, to the ever-increasing complexity of performing SEO on a website. Where once we dealt with title tags, meta descriptions, maybe a keyword tag, an H1 tag, and some on-page keyword placement, we now face a massively complex, interconnected web of factors, weights, and impacts.
And today's world of search lives on sliding scales. Item A matters X% if one condition exists. Change that condition and the percent changes, redefining the weight of Item A. And it's safe to say that today the entire alphabet (and then some) represents discrete elements across the multitude of algorithms in play.
In fact, Danny Sullivan from Google just recently alluded to this when he said, "...And of course, as we all know, there's multiple systems that all come into play…" Echoing this concept of complexity and highlighting the need to remain in touch with the bigger picture, Google's John Mueller said, "For a lot of the guidance that we have, it's a lot more now about the bigger picture, because it feels like from a technical point of view, things are often pretty reasonable."
You can use the concepts from Complexity Theory to help you explain to others in your organization, in more detail, how interrelated the factors impacting ranking are. How your program needs the required support, how teams need to work together and to help prioritize work items. Not every issue needs to be perfectly solved, but almost all issues need a solution.
Let's take a look at a couple of examples of how CT overlaps SEO work.