You are mentally strong, have a strong psyche and don't give up easy. You aren't as much stubborn as you are stable. The trait can be used for resisting torture, pain or in any form of mental or social combat. Whenever you make a save or skill check related to mental strength you may use this trait. The reason for your mental strength can be lifelong training, genetics or even frequent trauma. The reason matters little, the fact that you rarely lose your cool or crack under pressure does.