1 [countable] American EnglishSE
a day when a school or organization invites the public to come in and see the work that is done there:
Parents are invited to attend the open house next Thursday.
2 [uncountable] British English
if it is open house at someone's home, people are always welcome to visit at any time:
He kept open house for a wide range of artists and writers.
a party at someone's house that you can come to at any time during a particular period:
We're having an open house Sunday, noon to 5pm.
4 [countable] American EnglishBB
an occasion when someone who is selling their house lets everyone who is interested in buying it come to see it