plan ‧ e ‧ tar ‧ i ‧ um /ˌplænəˈteəriəm, ˌplænɪˈteəriəm $ -ˈter-/ BrE AmE noun [countable]
[ Date: 1700-1800 ; Language: Modern Latin ; Origin: Latin planeta ; ⇨ ↑ planet ]
a building where lights on a curved ceiling show the movements of planets and stars