nautical linear unit

NOUN
20 LETTERS · STARTS WITH N

nautical linear unit is a valid 20-letter word worth 20 points in Scrabble and 28 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
N1A1U1T1I1C3A1L1L1I1N1E1A1R1U1N1I1T1
1 + 1 + 1 + 1 + 1 + 3 + 1 + 1 + 0 + 1 + 1 + 1 + 1 + 1 + 1 + 0 + 1 + 1 + 1 + 1 =20 PTS
DOUBLE WORD → 40TRIPLE WORD → 60
MEANING

What does nautical linear unit mean?