What is an estuary? (From Conrad's Heart of Darkness)

A water passage where a saltwater sea tide meets a freshwater river current.

On the whole river there was nothing that looked half so nautical. He resembled a pilot, which to a seaman is trustworthiness personified. It was difficult to realize his work was not out there in the luminous estuary, but behind him, within the brooding gloom.

From Joseph Conrad's Heart of Darkness.