Cush is a name used in Egyptian, Akkadian, and Hebrew writings to refer to the countries along the Upper Nile River, south of Egypt. In a narrower sense, Cush was the area between the second and fourth cataracts (large waterfalls) of the Nile River. This is roughly the northern part of modern Sudan. This area was also known as ancient Nubia.
In general, the Old Testament uses the term in the narrower sense. The Greeks called this region Ethiopia, which later gave its name to the modern country of Ethiopia. (The modern country of Ethiopia is located farther to the south and east.)
Cush in the Book of Genesis
The meaning of "Cush" in the book of Genesis is not as clear. In the garden of Eden story, Cush seems to be in Mesopotamia (Genesis 2:13). This is the region of the Tigris and Euphrates rivers (verse 14). This "Cush" might be the same as Kassite (Cossaean), which was the name used for the Babylonian rulers who controlled Mesopotamia for about 500 years until the 12th century BC.
So, the Cush mentioned in Genesis 10:6–8 could refer to two different places: Nubia (verses 6–7) and Mesopotamia (verses 9–12). Another possibility is that the Cush in Genesis 2:13 and 10:8 could be Kish. This was a Mesopotamian city that was traditionally considered the seat of the first Sumerian dynasty after the great flood.
Cushites in the Bible
There is less confusion about the term "Cushite." With one possible exception in Numbers 12:1, Cushite always refers to people from Nubia, the African Cush.
The first messenger whom Joab, King David's military commander, sent to announce Absalom's defeat to David was a Cushite (2 Samuel 18:21–32). We can tell this messenger was a foreigner because he did not know about a shortcut and because he was not sensitive to David's feelings when delivering the message. The King James Version translates the Hebrew word as the name "Cushi," but most scholars think this is incorrect. Most English Bible versions translate the other occurrences of Cush and Cushite as Ethiopia and Ethiopians.
Moses had a wife who was known as a Cushite (Numbers 12:1). This description can be understood in several ways:
She could have been from Nubia, which would make her a second wife, different from Zipporah.
She could have been from Cushan, making her possibly a Midianite, perhaps the same person as Zipporah.
The term might refer to her darker skin and foreign origin, possibly but not necessarily referring to Zipporah.