Apparently “codependency” means something significantly different to what I guessed (which was interdependency, depending on each other). https://en.wikipedia.org/wiki/Codependency:

> In psychology, codependency is a theory that attempts to explain imbalanced relationships where one person enables another person's self-destructive behavior,[1] such as addiction, poor mental health, immaturity, irresponsibility, or under-achievement.[2]

> Definitions of codependency vary, but typically include high self-sacrifice, a focus on others' needs, suppression of one's own emotions, and attempts to control or fix other people's problems.[3]

I've heard codependency described as being dependent on others being dependent on you.

It probably has some rational basis in child rearing, it will benefit survival for the parents to be deeply dedicated to supporting their children, to the point where they receive a psychological reward from that dependency. But unhealthy when it comes to adult relationships, at least beyond a certain point.