say it: kah-luh-SEE-uhm
coliseum is a noun meaning "an oval large stadium with tiers of seats; an arena in which contests and spectacles are held". It carries one recorded sense.
an oval large stadium with tiers of seats; an arena in which contests and spectacles are held
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 coliseum 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.