The definition of dependency is the inability to function without someone else’s help and codependency is specifically an extreme emotional reliance on someone. Both dependency and codependency are seen as emotionally unhealthy because within those two states there is no room for autonomy. The only way dependency can thrive is if there is a lack of autonomy to begin with. It is important to make the distinction between dependency and being supportive.
Often people will give examples of addicts being in codependent relationships. That one addict needs another to justify their actions. Codependency can be extreme but it can also be very subtle. Either way in the long term any relationship of that sort can be emotionally harmful. A more subtle example of a codependent relationship is a person who needs their partner to need them for emotional support and will somehow hinder their partners progression towards autonomy.
Codependent relationships exist in friendships, romantic relationships and familial relationships. Money can also be used to further entrench a codependent relationship. For example a younger brother may rely on a older sibling to regularly give them handouts. The older sibling is frustrated by this but yet keeps on giving his little brother handouts. There is an aspect of being relied upon and needed by his younger brother that keeps them locked into this codependency. Thus stopping the younger sibling from trying to sort out their own finances.