also: てんおうざん
noun
decisive point; turning point; watershed moment
Metaphorical use derived from the historical Battle of Yamazaki (1582), where Mount Tennōzan was the strategic key. Used for a critical juncture in a competition, project, or series of events.
この試合が優勝への天王山だ。
This match is the decisive point on the way to the championship.
プロジェクトの天王山
The watershed moment for the project is next month's presentation.