instability line

16 LETTERS · STARTS WITH I

instability line is a valid 16-letter word worth 20 points in Scrabble and 24 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

I1N1S1T1A1B3I1L1I1T1Y4L1I1N1E1

1 + 1 + 1 + 1 + 1 + 3 + 1 + 1 + 1 + 1 + 4 + 0 + 1 + 1 + 1 + 1 =20 PTS

DOUBLE WORD → 40TRIPLE WORD → 60

MEANING

What does instability line mean?