The term originally came out of 12-step programs like AA. In that system there is the addict and then there is their partner who is the "co-dependent". This person is wrapped up in the addiction just as much as the addict. The addiction rules their life with equal fervor. On the surface people's sympathies often go with the co-dependent rather than the addict. We may find ourselves thinking "oh, that poor person, attached to that addict who is making their life so miserable".
But the reality is that both people are dysfunctional. The co-dependent person is getting some kind of perk out of being in this situation. Mabye it's the sympathy of others. Or the excuse as to why they can't get ahead in life-- because they are too busy taking care of the addict. The addict looks like the "hot mess" in the relationship and in contrast the co-dependent looks well put together.
In this type of relationship both people actually have an agenda. The addict wants to maintain their addiction and the co-dependent wants to have an excuse for why their life isn't the way they want it to be. In fact in these relationships both partners are using the relationship for meeting mostly their own needs. They do not expect the relationship to be fair. In fact, they expect and act as if relationships will NOT be fair! Their needs may be to have a partner hold down the home front while they engage in affairs or addictions. Or the need may be to play the martyr and support an addict while complaining to everyone else about how mistreated they are. This is not interdependency.
Interdependency involves the idea that in a relationship, we are greater than the sum of our parts. That two together can accomplish more than either one alone. Or as the african saying goes, "if you are going to take a short journey, go alone and you will go faster. But if you must go far, take another". Life is a long journey and taking a partner along with us to be our help-mate can be incredibly fulfilling and successful if we can learn the basic rules of truly mutual relationships. Psychologists and researchers call these truly mutual relationships "secure", meaning that each partner knows that the other person is going to be there for them no matter what. And that each partner also knows that whatever happens to their partner happens to them too. So in a secure relationship, one would never do anything that would intentionally hurt the other person, or be unfair to them, or take advantage of them, because in the end it would hurt BOTH of them. It is a state of knowing that you are bound together on all levels and acting accordingly.
The method of couples therapy that I practice, called the Psychobiological Approach to Couples Therapy, or PACT, talks a lot about how to develop a secure relationship. Dr. Diane Poole Heller interviewed PACT's originator, Dr. Stan Tatkin, about the phenomenon of interdependency. You may enjoy watching the interview to learn more about this:
I hope you find this video helpful as well as the information provided above. Remember, all relationships can change given proper support and guidance.