A room with tatami flooring, sliding doors, and traditional architectural features, as opposed to a Western-style room (洋間).
この旅館には日本間と洋間の両方があります。
This inn has both Japanese-style and Western-style rooms.
Compound of 日本 (Japan) and 間 (room). The exact historical derivation is straightforward; the term directly describes a room in traditional Japanese style.