Source : Webster's Revised Unabridged Dictionary (1913)

Self-possessed \Self"-pos*sessed"\, a.
   Composed or tranquill in mind, manner, etc.; undisturbed.

Source : WordNet®

self-possessed
     adj : in full control of your faculties; "the witness remained
           collected throughout the cross-examination"; "perfectly
           poised and sure of himself"; "more self-contained and
           more dependable than many of the early frontiersmen";
           "strong and self-possessed in the face of trouble"
           [syn: {collected}, {equanimous}, {poised}, {self-collected},
            {self-contained}]