1. a party or reception during which anyone who wishes may visit to share in a celebration, meet a special guest, etc.
2. a time during which a school, institution, etc., is open to the public for exhibition or for some specific occasion.
3. a house hospitably open to all friends who may wish to visit it.
4. keep open house, to be prepared to entertain visitors at any time: They keep open house for artists and writers.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.