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academic costume

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noun
  1. the ceremonial garb of the students and faculty in schools, colleges, and universities, consisting of a flat cap (mortarboard), a long, wide-sleeved gown, and sometimes a hood, worn especially at commencement exercises.
Also called academic dress.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for academic dress

academic dress

noun
  1. formal dress, usually comprising cap, gown, and hood, worn by university staff and students
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