defence mechanism

NOUN
17 LETTERS · 4 SYLLABLES · STARTS WITH D

defence mechanism is a valid 17-letter word worth 31 points in Scrabble and 36 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
D2E1F4E1N1C3E1M3E1C3H4A1N1I1S1M3
2 + 1 + 4 + 1 + 1 + 3 + 1 + 0 + 3 + 1 + 3 + 4 + 1 + 1 + 1 + 1 + 3 =31 PTS
DOUBLE WORD → 62TRIPLE WORD → 93
MEANING

What does defence mechanism mean?