Both 囲碁 and 碁 can be used. The former is more common, but the second isn’t uncommon, and is almost always the term used in compound words (e.g. in goban 碁盤).
Both 囲碁 and 碁 can be used. The former is more common, but the second isn’t uncommon, and is almost always the term used in compound words (e.g. in goban 碁盤).