Sega Enterprises is a Japanese company that founded the subsidiary Sega of America to market its products in North America. Sega Enterprises can trace its origins to Americans such as David Rosen who were in Japan after the war.
By the mid 80s, CSK, a Japanese software firm, purchased the company from Gulf and Western, the American owners at the time. They then established Sega of America, so I say "of Japan" to distinguish the Japanese office.
The name Sega, is derived from Service Games of Japan, the name in the 1950s.