a set of rules specifying the garb or type of clothing to be worn by a group or by people under specific circumstances: a military dress code; The restaurant's dress code requires men to wear jackets and ties at dinner.
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British Dictionary definitions for dress code
a set of rules or guidelines regarding the manner of dress acceptable in an office, restaurant, etc
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