plus-oneplus-ˈone noun [countable] informal 
someone who you take with you to an event when you have received an invitation for yourself and one other person, who is not named on the invitation: 
He asked her to come to the wedding with him as his plus-one.
someone who you take with you to an event when you have received an invitation for yourself and one other person, who is not named on the invitation: He asked her to come to the wedding with him as his plus-one.