noun
observation deck; viewing platform
A raised platform or deck, often in a tower, building, or scenic spot, designed for looking out over a view.
東京タワーの展望台から街を見下ろした。
I looked down at the city from the observation deck of Tokyo Tower.
この山の頂上
There is a viewing platform at the top of this mountain.
見晴らし台 is a more general term for a lookout point, often outdoors and less formal than 展望台, which is commonly used for structured decks in towers or buildings.
Compound of 展望 (てんぼう, 'view, outlook') + 台 (だい, 'platform, stand'). The reading だい is the standard rendaku form of 台 when used as a suffix.