group action

NOUN
12 LETTERS · STARTS WITH G

group action is a valid 12-letter word worth 16 points in Scrabble and 21 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
G2R1O1U1P3A1C3T1I1O1N1
2 + 1 + 1 + 1 + 3 + 0 + 1 + 3 + 1 + 1 + 1 + 1 =16 PTS
DOUBLE WORD → 32TRIPLE WORD → 48
MEANING

What does group action mean?