noun
Refers to a participant in a formal board game match, especially go or shogi. Not used for casual players or video games.
両対局者は深く一礼してから対局を始めた。
Both players bowed deeply and then began the game.
プレイヤー is a general term for a player in any game or sport, while 対局者 is limited to formal board game matches.
Compound of 対局 (taikyoku, 'a game of go, shogi, etc.') + 者 (sha, 'person').