dorsal fin

the fin or finlike integumentary expansion generally developed on the back of aquatic vertebrates.

1760–70 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
dorsal fin
any unpaired median fin on the backs of fishes and some other aquatic vertebrates: maintains balance during locomotion

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
It also has a thick layer of blubber and no dorsal fin so it can swim easily
  under the ice.
Each type is distinguished by dorsal fin shape, diet, and even language.
The nickname arises from its tendency to swim close to the surface, raising its
  dorsal fin above the water.
He had a weird growth along his dorsal fin, and that gape-mouth grimace you see
  in older fish.
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