Hi,Beth !
That's a very good question!
It's a Chinese idiom ,江山 from literally means :river and mountain. But actually that's means territory , when Chinese mention it as a slang ,that's has the other meaning , make a consequence for 1 time,it will not be make again. I will give you some examples as following:
1.既然双方都不服气,那么我们再比最后一次,一次定江山!
2.比赛开始了,因为这次是一次定江山,双方都进入了全力以赴的阶段!
3.这次职业技能大赛,一次定江山,请各位选手严格遵守比赛规则,使出看家的本事来!
4.这次竞标可是一次定江山,所有公司都在展示自己公司的最大优势,也不知道会花落谁家。
Hope that's helpful for you ! Thank you !
欢迎大家讨论补充~