NOUN (3)

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;

The Collaborative International Dictionary of English v.0.48:

Planetarium \Plan`e*ta"ri*um\, n. [NL.: cf. F. plan['e]taire.
   See Planetary.]
   An orrery. See Orrery.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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