USSR

This term refers to a specific historical entity and is used as a proper noun. It carries a heavy political and historical weight, often evoking themes of the Cold War, communism, and the geopolitical divide between the East and West during the mid-to-late 20th century.

Because it is a proper noun representing a single, unique state, it is used exclusively in the singular. It does not have a plural form and is not used as a common noun.