self-possession

NOUN /sɛlfpə'zɛʃən/
15 LETTERS · 3 SYLLABLES · STARTS WITH S

self-possession is a valid 15-letter word worth 19 points in Scrabble and 22 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
S1E1L1F4-0P3O1S1S1E1S1S1I1O1N1
1 + 1 + 1 + 4 + 0 + 3 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 =19 PTS
DOUBLE WORD → 38TRIPLE WORD → 57
MEANING

What does self-possession mean?