Emotional Boundary
An emotional boundary is the line between your feelings and someone else's. It means you stay responsible for your own emotions and let other people stay responsible for theirs.
A friend calls, upset, and within a minute your whole evening tilts toward fixing how she feels. That is an emotional boundary going soft. With one in place you can care about her without taking on the job of making the feeling go away.
Emotional boundaries get blurry early, often in a home where a child had to track a parent's mood to stay safe. The skill of feeling responsible for everyone in the room is intelligent and useful at the time. Later it shows up as guilt when someone near you is unhappy, an urge to manage other people's reactions, and a habit of treating their disappointment as proof you did something wrong.
Holding an emotional boundary sounds like staying warm while letting a feeling belong to the person who has it. You can listen without rescuing. You can hear someone is angry and still not change your answer. The other person's feelings are real and they are theirs.