noun
ko threat
A move in the game of Go that creates a threat elsewhere on the board, forcing the opponent to respond and allowing the player to retake the ko.
コウ材がなければ、コウに勝てない。
Without a ko threat, you cannot win the ko.
彼はコウ材を打ってから、すぐにコウを取り返した。
He played a ko threat and then immediately took the ko back.
コウ is the ko rule itself; コウ材 is a move used as a threat to resolve a ko situation.
コウ立て refers to the act of setting up or initiating a ko threat, while コウ材 is the threat move itself.
Compound of コウ (ko, from the Go term for the ko rule) and 材 (ざい, material). The term literally means 'ko material', referring to a move used as a resource in a ko fight.