Other forms: tipping points
A tipping point is a moment at which a change or effect becomes irreversible. After you reach the tipping point, there's no going back.
We often think of climate tipping points these days — changes that reach a threshold past which there's no way to stop their effects on Earth's climate. If melting permafrost reaches a tipping point, it means so much has melted, releasing methane gas that further warms the atmosphere, that there's no way that ice will ever form again. Tipping point literally meant "point at which an object tips over" until it began being used figuratively around 1950.