The word ''Zaire'' is from a Portuguese adaptation of a Kikongo word ''nzadi'' ("river"), a truncation of ''nzadi o nzere'' ("river swallowing rivers"). The river was known as ''Zaire'' during the 16th and 17th centuries; ''Congo'' seems to have replaced ''Zaire'' gradually in English usage during the 18th century, and ''Congo'' is the preferred English name in 19th-century literature, although references to ''Zaire'' as the name used by the natives (i.e. derived from Portuguese usage) remained common.