IN THE LAST forty years, “networks” have become increasingly familiar metaphors for the way our world works. The term “networking” first existed to describe business professionals exchanging information and opportunities, and in the last two decades, the rapid growth of social media brought the concept of a “social network” to the forefront of public consciousness and transformed the way we interact with one another. We codify relationships with one another by friending and following and let algorithms suggest new connections based on our “mutuals.” A romantic relationship being “Facebook Official” is now a socially significant marker of commitment. Online social networks are critical to the organizing of social and political movements. In very recent years, network analysis has come into the public eye in the form of “contact tracing,” a means of managing the spread of the COVID-19 virus.

The network metaphor is everywhere—but what, exactly, is a network? At its core, a network is a way of organizing a set of data based on how each piece is connected to the other pieces. For example, a social network organizes a set of people by describing the relationship one person has to another person, as in Figure 1.