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anal fin

noun
  1. the median, unpaired fin on the ventral margin between the anus and the caudal fin in fishes.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for anal fin

anal fin

noun
  1. a median ventral unpaired fin, situated between the anus and the tail fin in fishes, that helps to maintain stable equilibrium
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012