heat unit

NOUN /hit_'junɪt/
9 LETTERS · STARTS WITH H

heat unit is a valid 9-letter word worth 11 points in Scrabble and 12 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
H4E1A1T1U1N1I1T1
4 + 1 + 1 + 1 + 0 + 1 + 1 + 1 + 1 =11 PTS
DOUBLE WORD → 22TRIPLE WORD → 33
MEANING

What does heat unit mean?