Composure

Composure measures willpower, your ability to keep it together and your general mental resilience.
You will generally encounter Composure checks when facing very stressful situations, such as dropping to 0 hit points or seeing a very gruesome sight. Failling such a check can result in you getting a point of Stress. In addition, you can in some cases add your Composure modifier to saving throws against being charmed or frightened. A successful Composure check could even save you from going mad.