/ fɪn; NAmE / noun
1.
a thin flat part that sticks out from the body of a fish, used for swimming and keeping balance
—picture at fish
2.
a thin flat part that sticks out from the body of a vehicle, an aircraft, etc., used for improving its balance and movement :
tail fins
—picture at plane
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WORD ORIGIN
Old English finn , fin , of Germanic origin; related to Dutch vin and probably ultimately to Latin pinna feather, wing.