a person who accompanies someone to a party or other event when the invitation allows the invited guest to bring another person: I phoned Tom to ask if he would be my plus-one at the gala.
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How to use plus-one in a sentence
The length should be the headsize measurement plus one-half inch.Make Your Own Hats | Gene Allen Martin
British Dictionary definitions for plus-one
informal a person who accompanies an invited person to a social function
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