Used to add a condition, exception, or limitation to a preceding statement. Often introduces a proviso or caveat.
We will go on the excursion tomorrow. However, if it rains, it will be cancelled.
この部屋は自由に使ってください。ただし、後片付けはしてください。
Please feel free to use this room. Provided that you clean up afterwards.
Admission is free. However, eating and drinking are prohibited.