Named after Clapham, though the station is actually in Battersea.
It opened in 1863 where several railway lines meet, hence Junction. Battersea then had market gardens but also a chemical works and a lime kiln, while Clapham, about a mile away, had become popular with wealthy Londoners. The owners thought Clapham Junction sounded more respectable. 'Clapham' itself means the homestead or enclosure near a hill.