to decide to do something important or risky, especially after thinking about it for a long time:
We took the plunge and set up our own business.
a sudden large decrease in the price, value etc of something
3 [countable usually singular]
a sudden movement down or forwards:
The plane began a headlong plunge towards the Earth.
4 [countable usually singular]
when someone suddenly becomes involved in something new
his sudden plunge into marriage
5 [countable usually singular]
a jump or dive into water, or a quick swim
a quick plunge in the lake