large demonstration board (for Go, Shogi, etc.)
A large vertical board used to display game positions to an audience during lectures, tournaments, or TV broadcasts.
プロ棋士が大盤を使って解説した。
The professional player gave a commentary using the large demonstration board.
Compound of 大 (large) and 盤 (board). The reading おおばん follows standard kun'yomi for 大 and on'yomi for 盤.