Real Dictionary

watercress
uncountable noun: Watercress is a small plant with white flowers which grows in streams and pools. Its leaves taste hot and are eaten raw in salads.

watercress in American English
a white-flowered plant (Nasturtium officinale) of the crucifer family, growing generally in running water, as from springs: its leaves are used in salads, soups, etc.
noun: a cress, Nasturtium officinale, of the mustard family, usually growing in clear, running streams and having pungent leaves

watercress in British English
noun: an Old World plant, Nasturtium officinale, of clear ponds and streams, having pungent leaves that are used in salads and as a garnish: family Brassicaceae (crucifers)