Final answer:
A large structure for open-air sports or other entertainments is called a stadium or an amphitheatre, with the Colosseum being a prime example of the latter.
Step-by-step explanation:
A large structure for open-air sports or other entertainments is typically called a stadium or an amphitheatre. The Colosseum, also known as the Flavian Amphitheatre, is one of the most iconic examples of an ancient amphitheatre and was used for various public spectacles such as gladiatorial contests and mock sea battles. This historical structure could accommodate up to 50,000 to 80,000 spectators and featured innovative design elements like numbered entrances and an underground staging area, making it similar to modern open-air sports venues in terms of functionality and audience capacity.