A shared house occupied by non-Japanese in urban Japanese centres such as
Tokyo,
Osaka, and
Kyoto.
Rooms are generally rented on a weekly or monthly basis, and it is pretty rare for a foreigner to stay much longer than three months.
The vast majority of gaijin house residents are foreign workers, who use the facilities as half-way houses until more permanent (and private) accomodation can be found.
Gaijin houses are advertised in the foreign-language press (usually free weekly inkies) in all major cities.
BYO Domestos and double up on the flax -- these places can be somewhat unhygenic...