noun
A critical moment or juncture where a significant change occurs.
あの試合の逆転ホームランが一点機だった。
That come-from-behind home run was the turning point of the game.
The term is composed of 一 (one), 点 (point), and 機 (opportunity/chance), literally 'one-point opportunity'. The exact historical derivation is uncertain, but it is used to describe a pivotal moment.