unidirectional signal

21 LETTERS · STARTS WITH U

unidirectional signal is a valid 21-letter word worth 24 points in Scrabble and 32 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

U1N1I1D2I1R1E1C3T1I1O1N1A1L1S1I1G2N1A1L1

1 + 1 + 1 + 2 + 1 + 1 + 1 + 3 + 1 + 1 + 1 + 1 + 1 + 1 + 0 + 1 + 1 + 2 + 1 + 1 + 1 =24 PTS

DOUBLE WORD → 48TRIPLE WORD → 72

MEANING

What does unidirectional signal mean?