noun
go stone; go piece
Refers to the black and white stones used in the board game go (囲碁).
See also: 囲碁
碁石を一つ盤に置いた。
I placed a go stone on the board.
囲碁 is the game of go itself, while 碁石 refers specifically to the stones used as playing pieces.
Compound of 碁 (go, the board game) and 石 (stone).