Adjective : imperturbable composure.    From Dictionary.com. 

A certain degree of mental repose -- or what may possibly be called imperturbableness -- is necessary to influence.    From Wordnik.com.  [Lessons in Life A Series of Familiar Essays] Reference

I had nothing in my favour to balance this, but a sort of constitutional equanimity and imperturbableness of temper, which, if I was at any time silenced, made me not look like a captive to be dragged at the chariot-wheels of my adversary.    From Wordnik.com.  [Thoughts on Man: His Nature, Productions, and Discoveries] Reference