What is Firth-of-clyde?

Firth-of-clyde definition and meaning on Dictionary terms:

a river in S Scotland, flowing NW into the Firth of Clyde. 106 miles (170 km) long.
Firth of, an inlet of the Atlantic, in SW Scotland. 64 miles (103 km) long.
a male given name: a Scottish family name, after the Clyde River.