A boundary is a barrier; something that separates two things. Walls, fences and cell membranes are examples of physical boundaries. Psychological boundaries can be said to exist too, even though such boundaries have no physical reality. Psychological boundaries are constructed of ideas, perceptions, beliefs and understandings that enable people to define not only their social group memberships, but also their own self-concepts and identities. Such boundaries are the basis by which people distinguish between “We” or “I” (group members; insiders; part of “Us”) and “Other” (outsiders and examples of what is “not-self”). Each person can be said to have a psychological identity boundary around themselves by which they distinguish themselves from other people. Like other boundaries, this identity boundary both separates people and also defines how they are linked together. This is to say that the act of drawing the boundary itself provides the basis for saying that one person is separate from another psychologically, but does so only by drawing a distinction between those two people, which implies a relationship, never the less. Self cannot exist without also “Not-self” existing, just as figure cannot exist without ground against which to contrast. Identity necessarily includes social relationships which are built into the self to varying degrees.
https://www.mentalhelp.net/psychotherapy/boundaries-and-dysfunctional-family-systems/
