vision defect

NOUN
13 LETTERS · STARTS WITH V

vision defect is a valid 13-letter word worth 21 points in Scrabble and 24 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
V4I1S1I1O1N1D2E1F4E1C3T1
4 + 1 + 1 + 1 + 1 + 1 + 0 + 2 + 1 + 4 + 1 + 3 + 1 =21 PTS
DOUBLE WORD → 42TRIPLE WORD → 63
MEANING

What does vision defect mean?