An open convention party sponsored by fans who are bidding against each other for the right to host a future convention. The idea is to promote good will (and possibly secure votes) for their bid. You'll find them frequently at Worldcons, often years in advance of the actual voting, as well as at other regional and local conventions under the assumption that the attendees of these smaller conventions might also attend the Worldcon to cast their ballots. But also see Minneapolis in '73.
Contributors: Dr. Gafia