Sega went through a number of buyouts and merges and renames and spinoffs over the decades so it's a pretty complex history to follow as it alternated between American and Japanese ownership.

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.