Cochinchina

Cochinchina is a historical exonym for part of Vietnam, depending on the contexts. Sometimes it referred to the whole of Vietnam, but it was commonly used to refer to the region south of the Gianh River.

In the 17th and 18th centuries, Vietnam was divided between the Trịnh lords to the north and the Nguyễn lords to the south. The two domains bordered each other on the Son River. The northern section was called Tonkin by Europeans, and the southern part, Đàng Trong, was called Cochinchina by most Europeans and Quinam by the Dutch.[2]

Jean-Louis Taberd, in his 1838 map, called Tonkin as "Cocincina exterior" (Đàng Ngoài) and "Cochin China"[a] as "Cocincina interior" (Đàng Trong). In this classic 1838 map, the Gianh River is north of "Lũy Sầy" (an incorrect pronunciation and spelling of "Lũy Thầy"[b]) demarcating "Cocincina exterior" (or "Outer Annam") from "Cocincina interior" (or "Inner Annam"). A small river immediately north of "Lũy Sầy", drawn but not annotated, was likely the Son River, a tributary to the Gianh River.

https://en.wikipedia.org/wiki/Cochinchina