imperfect tense

NOUN
15 LETTERS · STARTS WITH I

imperfect tense is a valid 15-letter word worth 23 points in Scrabble and 27 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
I1M3P3E1R1F4E1C3T1T1E1N1S1E1
1 + 3 + 3 + 1 + 1 + 4 + 1 + 3 + 1 + 0 + 1 + 1 + 1 + 1 + 1 =23 PTS
DOUBLE WORD → 46TRIPLE WORD → 69
MEANING

What does imperfect tense mean?