invisible ink

13 LETTERS · STARTS WITH I

invisible ink is a valid 13-letter word worth 21 points in Scrabble and 26 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

I1N1V4I1S1I1B3L1E1I1N1K5

1 + 1 + 4 + 1 + 1 + 1 + 3 + 1 + 1 + 0 + 1 + 1 + 5 =21 PTS

DOUBLE WORD → 42TRIPLE WORD → 63

MEANING

What does invisible ink mean?