A hybrid event is a tradeshow, conference, unconference, seminar, workshop or other meeting that combines a "live" in-person event with a "virtual" online component.
They also enable participation by people who might be unable to attend physically due to travel or time zone constraints, or through a wish to reduce the carbon footprint of the event.
The open, participatory nature of unconferences (e.g., Barcamp) and their focus on sharing content, makes them hybrid events too.
Generally, the virtual component involves an online representation of the live event.
As well as allowing a physical event to reach a wider audience, these online tools also provide a means for physical attendees to interact with each other, with the event organisers and with online participants, and for online participants to interact with each other.