Source : Webster's Revised Unabridged Dictionary (1913)

Planetarium \Plan`e*ta"ri*um\, n. [NL.: cf. F. plan['e]taire.
   See {Planetary}.]
   An orrery. See {Orrery}.

Source : WordNet®

planetarium
     n 1: a building housing an instrument for projecting the
          positions of the planets onto a domed ceiling
     2: an optical device for projecting images of celestial bodies
        and other astronomical phenomena onto the inner surface of
        a hemispherical dome
     3: an apparatus or model for representing the solar systems
     [also: {planetaria} (pl)]