a thin flat part that sticks out from the body of a fish, used for swimming and keeping balance （鱼的）鳍 
a thin flat part that sticks out from the body of a vehicle, an aircraft, etc., used for improving its balance and movement 鳍状物，翼（车辆、航空器等用以保持平衡等的突出窄扁部分） 
tail fins 垂直尾翼
Word OriginOld English finn, fin, of Germanic origin; related to Dutch vin and probably ultimately to Latin pinna ‘feather, wing’.