Yeah, it’s a badly chosen word that in plain common-sense etymology would seem to mean “reciprocally dependent” or “interdependent”, which is a good thing, but that’s not what it means as used in clinical social psych.
“Codependency” in that sense is participation in a pathological matrix of behaviors where each individual’s participation perpetuates the matrix by eliciting the behaviors of the other participant(s).
Example: Woman feels unloved by her husband who doesn’t pay much sexual attention to her / rather than acknowledge her emotional state directly she experiences it as heightened awareness of whatever physical misery she feels, and tension causes her back to go into spasms / she complains of horrible backaches and mysterious disabling pains / he is attentive and sympathetic / she feels more reassured about the love but hates the pain and can’t do things / as a consequence of thinking of her as an invalid in pain he is less inclined to think of her sexually or act on it if he does / he is concerned about her but annoyed about her being an invalid and about them hardly ever having sex any more / he feels bad about being dissatisfied with the relationship because she’s got those horrid back problems so he doesn’t talk about his frustrations and just distances himself instead.
Each person’s pathological self-hurting participation is a response to the other person’s pathological self-hurting participation so this matrix of behavior depends on both of them playing into it. No one benefits from it, but they remain trapped in its cycle, unaware of what’s really going on.
You can get considerably more than just two people wrapped up in one of these, especially in families!