(astronomy) The extent of a universe within the cosmological horizon of an observer.

plural of observable universe

a lookout or vantage point

Ferris wheel

plural of observation wheel

plural of observation car

(rail transport) A railway passenger car made so as to facilitate seeing the scenery en route, sometimes with glass sides or with a kind of open balcony at the rear.

plural of observation post

plural of observation deck

A rudimentary building used by people making field observations.