Meaning

noun a building housing an instrument for projecting the positions of the planets onto a domed ceiling

noun an optical device for projecting images of celestial bodies and other astronomical phenomena onto the inner surface of a hemispherical dome

noun an apparatus or model for representing the solar systems

originBorrowed from New Latin planētārium. Equivalent to planet + -arium.

Etymology from Wiktionary, CC BY-SA 3.0