volume unit

NOUN
11 LETTERS · STARTS WITH V

volume unit is a valid 11-letter word worth 15 points in Scrabble and 21 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
V4O1L1U1M3E1U1N1I1T1
4 + 1 + 1 + 1 + 3 + 1 + 0 + 1 + 1 + 1 + 1 =15 PTS
DOUBLE WORD → 30TRIPLE WORD → 45
MEANING

What does volume unit mean?