playing field boundary (competitive karuta)
Refers specifically to the boundary line of the playing field in competitive karuta (a Japanese card game).
競技線から出たら反則になる。
If you go out of the playing field boundary, it's a foul.
選手は競技線の内側に座る。
The players sit inside the playing field boundary.
Compound of 競技 (kyōgi, 'competition') and 線 (sen, 'line'), literally 'competition line'. The term is specific to competitive karuta.