The Collaborative International Dictionary of English v.0.48:

Self-possession \Self`-pos*ses"sion\, n.
   The possession of one's powers; calmness; self-command;
   presence of mind; composure.
   [1913 Webster]

WordNet (r) 3.0 (2006):

self-possession
    n 1: the trait of resolutely controlling your own behavior [syn:
         self-control, self-possession, possession,
         willpower, will power, self-command, self-will]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "self-possession":
   aplomb, assurance, ataraxy, balance, calmness, composure,
   confidence, constraint, control, coolness, discipline, equability,
   equanimity, equilibrium, imperturbability, independence,
   level head, levelheadedness, phlegm, poise, possession,
   presence of mind, restraint, sangfroid, self-assurance,
   self-command, self-confidence, self-conquest, self-control,
   self-denial, self-discipline, self-government, self-mastery,
   self-restraint, well-regulated mind