Go apprentice
An insei is an official student in Japan studying Go, seeking to become a professional player. Insei study at the Nihon Ki-in (Japanese Go Institute), and once a year 3-4 players who qualify in the yearly qualification tournament become professional players.
Asian players are required to become a professional before the age of 18 while studying as insei, but foreigners have the chance up to the age of about 30.
To become an insei, a person requires a professional to sponsor them, and an application to the Nihon Ki-in. There is no official way to contact a professional for sponsorship.
Additional Notes
An insei in Korea is a kenkyuusei.