piece
A piece in board games is a unit that occupies, moves through, or is placed onto the board as part of play [@britannica2026chess; @britannica2026go].
Pieces matter because they turn the board from a passive surface into an active position. A board without pieces is only a structure. Once pieces enter it, the board becomes a changing field of claims, threats, routes, and constraints. In chess the differentiated pieces create distinct movement powers, while in weiqi the stones gain meaning from placement and relation rather than from piece type [@britannica2026chess; @britannica2026go].