a person who abstains from alcoholic beverages at a gathering in order to be fit to drive companions home safely.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for designated driver
a person who volunteers not to drink alcohol on a social occasion, so that he or she can safely drive other people who have been drinking
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