say it: plan-uh-TER-ee-uhm
planetarium is a noun meaning "a building housing an instrument for projecting the positions of the planets onto a domed ceiling". It carries 3 recorded senses.
a building housing an instrument for projecting the positions of the planets onto a domed ceiling
an optical device for projecting images of celestial bodies and other astronomical phenomena onto the inner surface of a hemispherical dome
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for planetarium come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.