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isophone

British  
/ ˈaɪsəˌfəʊn /

noun

  1. linguistics an isogloss marking off an area in which a particular feature of pronunciation is found

"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

Etymology

Origin of isophone

C20: from iso- (as in isogloss ) + -phone (as in phoneme )