A Western-style room (usually with wooden/raised floors and Western furniture such as beds and chairs), as opposed to a Japanese-style room.
ホテルは洋室と和室から選べます。
You can choose between a Western-style room and a Japanese-style room at the hotel.
この家は和室が二つ、洋室が一つある。
This house has two Japanese-style rooms and one Western-style room.
子ども部屋を洋室にリフォームした。
We remodeled the child's room into a Western-style room.