manic depressive illness

NOUN
24 LETTERS · STARTS WITH M

manic depressive illness is a valid 24-letter word worth 32 points in Scrabble and 40 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
M3A1N1I1C3D2E1P3R1E1S1S1I1V4E1I1L1L1N1E1S1S1
3 + 1 + 1 + 1 + 3 + 0 + 2 + 1 + 3 + 1 + 1 + 1 + 1 + 1 + 4 + 1 + 0 + 1 + 1 + 1 + 1 + 1 + 1 + 1 =32 PTS
DOUBLE WORD → 64TRIPLE WORD → 96
MEANING

What does manic depressive illness mean?