Source : Webster's Revised Unabridged Dictionary (1913)

Self-possession \Self`-pos*ses"sion\, n.
   The possession of one's powers; calmness; self-command;
   presence of mind; composure.

Source : WordNet®

self-possession
     n : the trait of resolutely controlling your own behavior [syn:
         {self-control}, {possession}, {willpower}, {self-command},
          {self-will}]