Composure and calm in stressful conditions; equilibrium.
1) The man's equanimity was remarkable, especially in the stressful conditions.
2) Equanimity is helpful in situations such as standardized test-taking, where being calm helps people focus on the exams.
This photo is for the definition of "equilibrium", a calm state of mind.
How to be Calm: How to Stay Calm in Stressful Situations
This person is talking about how to be calm in stressful situations.