Symbiosis is a concept which helps to explain one type of unhealthy relationship. The term symbiosis comes from biology, where it is used to describe two organisms working together for mutual benefit and, in the process, acting as one. (If you're a Star Trek fan, you'll know all about symbiosis. The Trill are a "joined" species, where two organisms work together as one. They turned up in Star Trek New Generation and later on in Deep Space Nine.)
The classical example of symbiosis is lichens, "plants" which grow on rocks or tree trunks. They build flat and often round colonies on bare stones, sometimes in beautiful colors like bright yellow or orange. Sometimes one can see them hanging off tree branches like beards. Lichens are "double-organisms". One part of them is an algae, which can produce food through photosynthesis. However, it needs water to live and it wouldn't survive living on bare, exposed rock. The other part of a lichen is a fungus. It can't produce food, because fungi can't photosynthesize, but it is very tough and can protect the algae from dehydration and other environmental dangers. The two organisms work as one and need each other to survive.
Psychologically, we use the term symbiosis in a similar way to describe a relationship where two people function as one. However, in contrast to the biological term, it refers to a relationship pattern which is not healthy, since a couple is existentially two separate people who need to be separate for both of them to be able to express their individuality and different needs.
Symbiosis can best be explained with the use of the ego state model. Picture two people. Both of them have three ego states, a parent ego state, an adult ego state and a child ego state. A healthy relationship can be described as one where both people can use all their ego states to relate to the other person. This means there is flexibility in the relationship. One person might be looking after the other for a while using their parent ego state while the other receives the care from a child ego state place. Then they go on to talk about daily routines, both using their adult ego states. And in the end, when matters are clarified, they might go on to play with each other, both accessing their child ego states.
In a relationship with a symbiotic pattern, both people use only some of their ego states to relate to each other, resulting in less flexibility. It's as if both partners take on stable roles and don't come out of them again. In symbiosis, two people function as if they only had one set of ego states between them. For example, person A might use their parent ego state and adult ego state to relate to person B, who mostly uses his or her child ego state to relate to A. Between them they only have one parent, one adult and one child ego state that is activated. This results in stable roles of A being the "carer" or the "responsible one", and B gets to be looked after. The same pattern will also result in a power differential between both partners. A gets to say what will happen, and B consents and follows. Or there might be a pattern , where B normally gets his or her way by using child-like tactics such as emotional blackmail or tantrums.
Both partners lose out in this pattern. Person A often gets power and can feel needed (for some people that will be part of their script), but they will miss out on being looked after or looking after themselves properly, because they don't access their child ego state and don't go with what they need and want for themselves. Person A might also not get a lot of time to play, but might always feel responsible for what is going on. Person B will get looked after, but that can also be experienced as belittling and not allowing person B to own their own power and competency. Person B doesn't access his or her adult and parent ego states and stays in a place of passive dependency.
The symbiotic pattern results in the classical set-up of a rescuer or caretaker and a needy and dependent partner in a relationship. It doesn't allow for flexibility or equality and it limits both partners in their freedom to be themselves.
However, both partners may have an investment in keeping the symbiosis going. Symbiotic relationships can be extremely stable and feel like they are very close, because they don't allow for difference. The roles are very predictable and therefore might feel very safe. Both partners know what's expected of them. Also, the roles in the symbiosis are learned in childhood. Person A might have started to be an emotional carer for his or her parents, when he or she was still a small child. Staying with this role as an adult allows him or her to stay within their script. The same is true for person B. He or she might have learned that it's best to stay little and not take responsibility or want his or her own way and staying within this role in an adult relationship means they don't have to change and look at themselves.
The way out of symbiosis starts when you look at what your investment in it is. Do you need to be needed? Do you need to be in control to feel safe (like person A)? Or do you get scared of being yourself or going out into the world? Or you may still have a lot of longing for being a child left over from your childhood and instead of facing the grief over your lost childhood you might opt for becoming the dependent person in an adult relationship (like person B). Remember that both people miss out on some of their innate capacities: person A on looking after themselves and going for what's good for them, and person B on their own power and competence. Whichever role you tend to go for, start owning what you are missing out on. Person A needs to accept that they are only emotionally responsible for themselves, like any other adult in this world. This may bring up a lot of issues for them, for example if being needed covers up a fear of being abandoned or a sense of not being good enough for the other person to stay around if they aren't looking after them. Person B needs to own and express their knowledge, competence and power in the world. He or she needs to take responsibility for their own emotional and physical well being.
If you want to know more about how symbiosis might influence your relationship look at Applications of symbiosis. Also, you will find more info on this under destructive relationship patterns.