defense mechanism

NOUN /dɪ'fɛns_'mɛkə,nɪzəm/
17 LETTERS · STARTS WITH D

defense mechanism is a valid 17-letter word worth 29 points in Scrabble and 33 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
D2E1F4E1N1S1E1M3E1C3H4A1N1I1S1M3
2 + 1 + 4 + 1 + 1 + 1 + 1 + 0 + 3 + 1 + 3 + 4 + 1 + 1 + 1 + 1 + 3 =29 PTS
DOUBLE WORD → 58TRIPLE WORD → 87
MEANING

What does defense mechanism mean?