An opera house was a facility specifically designed to hold performances of opera in.
The Teatro alla Scala in Milan, Italy was Earth's most famous opera house. (VOY: "Virtuoso")
The Sydney Opera House in Sydney was Australia's most famous and distinctive opera house. (LD: "Crisis Point 2: Paradoxus")
When The Doctor had been lost on Kelemane's planet in 2376, Chakotay suggested him to be searched in the areas of the Central Protectorate where opera houses or concert halls were located. The cultural center of the state containing such establishments ran along the shore of the Eastern Lake, where The Doctor was located. (VOY: "Blink of an Eye")
When Tom Paris created a holographic version of the Palace Theater in 2377, B'Elanna Torres at first mistook it for a lecture hall or an opera house. (VOY: "Repression")