So this clears that up:
Do not imagine that the term “South China Sea” ever implied Chinese ownership. It is a Western construction that dates to about 1900. Previously, European maps referred to it as the China Sea, and before that as part of the Indian Sea. When the Portuguese arrived there in the early sixteenth century they called it the Cham Sea, after the maritime kingdom of coastal Vietnam. Other names at various times include Luzon Sea and (by early Arab traders) the Clove Sea. To China it has long been the South Sea and to Vietnamese the East Sea. The Philippines now refers to it as the West Philippine Sea.
From Philip Bowring, Indonesia and China: The Sea Between in the New York Review of Books online.