A plus one is a guest. Someone you invite to come with you to a party or event

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This is used mostly in weddings, graduation ceremonies, or in big events. It means you can bring along one more guest with you to the event, most commonly as a date. You can bring "plus one" to an Awards ceremony and bring your mom. That works too. "She is my plus one" means she is your date for that event. Or she is accompanying to the event.