describes the degree of mental balance.
Emotionally stable people are calm, self-confident and level-headed. They can perceive their feelings quite intensively, but do not let them determine them or throw them off course. They are able to act rationally even in emotionally challenging situations. Overall, they can filter, analyze and control their emotions better than unstable (vulnerable) people.
In the sense of the Big Five personality model, the counterpart would be "neuroticism", whereby this is not to be understood in the Freudian sense, but merely expresses how much one is guided by negative feelings. Emotional stability means being able to withstand stress and pressure situations, low blows and fear and, in the best case, even being able to draw positive energy from them.