central building; central wing
Refers to the main or central building of a complex, such as a library, museum, or hospital. Often part of a proper name.
The library's central building is near the station.
This museum is divided into the central wing and the east wing.
別館 is an annex or separate building, contrasting with the central/main building.
Compound of 中央 (central) and 館 (building, hall). The exact historical derivation is uncertain, but the word follows a common pattern for naming institutional buildings.