The ancient outdoor theater in Rome is called the Colosseum. In all other contexts, the word for a large amphitheater used for sports or entertainment events is coliseum. We find claims that colosseum is the British spelling for the common noun, but this appears false, at least in current news publications that publish online. In fact, coliseum is preferred throughout the English-speaking world.
Examples
Here are a few examples of coliseum from around the world
Voters, in a rare midsummer referendum, rejected a plan that would have replaced the 39-year-old coliseum … [Star-Ledger (New Jersey)]
By no means everything about Stoke is quite so primitive, but this is certainly becoming a real coliseum for visiting teams. [Independent]
The children, first taken to local shelters, were later moved to a historic fort and then to a domed coliseum on the fairgrounds in San Angelo. [Sydney Morning Herald]
Irving said there are too many problems with the coliseum to justify the city’s demands for more money … [CBC]
And when Colosseum appears, it usually refers to the building in Rome—for example:
Their wedding was held in the Santa Francesca Romana church in Rome, near the Colosseum … [New York Times]
The Colosseum, the largest amphitheatre built by the Romans, is one of Italy’s most famous sites … [BBC]
