Original: Taking its name from the River Mersey, the title "Merseyside" came into existence as a metropolitan county in 1974, after the passage of the Local Government Act 1972, and the county consists of five metropolitan boroughs adjoining the Mersey Estuary, including the City of Liverpool.
Simplified: Taking its name from the River Mersey, the title "Merseyside" came into existence as a metropolitan county in 1974.