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ball (for sport, games, etc.)
kemari; type of football played by courtiers in ancient Japan
「球」 has other readings with different meanings.
子供たちは公園で球を蹴って遊んだ。
The children played kicking a ball in the park.
地球は完全な球ではなく楕円形だ。
The Earth is not a perfect sphere.
彼はガラスの球をコレクションしている。
He collects glass balls.