observation deck; observation room; observatory
Refers to an indoor space, often on a high floor of a building, designed for viewing scenery. Commonly found in towers, skyscrapers, and airports.
I could see Mt. Fuji from the observation deck of Tokyo Skytree.
We watched planes take off and land from the airport's observation room.
Compound of 展望 (てんぼう, 'view, outlook') and 室 (しつ, 'room').