means A building or institution equipped for watching and recording natural phenomena, especially the stars, planets, and weather.

from From Latin observare, 'to watch over, attend to' — itself ob- ('toward') plus servare ('to keep, guard, watch'). The same root gives us 'observe' and 'observant.' The -ory ending marks a place where a thing is done (as in 'laboratory,' 'dormitory'), so an observatory is, quite literally, a 'watching-place.' The word entered English in the 17th century, around the time astronomers were building serious houses for their telescopes.