simple interval

15 LETTERS · STARTS WITH S

simple interval is a valid 15-letter word worth 21 points in Scrabble and 27 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

S1I1M3P3L1E1I1N1T1E1R1V4A1L1

1 + 1 + 3 + 3 + 1 + 1 + 0 + 1 + 1 + 1 + 1 + 1 + 4 + 1 + 1 =21 PTS

DOUBLE WORD → 42TRIPLE WORD → 63

MEANING

What does simple interval mean?