colour vision deficiency

NOUN
24 LETTERS · STARTS WITH C

colour vision deficiency is a valid 24-letter word worth 38 points in Scrabble and 45 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
C3O1L1O1U1R1V4I1S1I1O1N1D2E1F4I1C3I1E1N1C3Y4
3 + 1 + 1 + 1 + 1 + 1 + 0 + 4 + 1 + 1 + 1 + 1 + 1 + 0 + 2 + 1 + 4 + 1 + 3 + 1 + 1 + 1 + 3 + 4 =38 PTS
DOUBLE WORD → 76TRIPLE WORD → 114
MEANING

What does colour vision deficiency mean?