In stock market, the word plunge is referred often when the market fall happens.
Generally the stock market fall is measured and determined using the respective points of that particular market.
For example, sensex points , nifty points etc.
If the sensex points falls at a particular day, due to some events happened in or around the market , then you can see the headlines at the end of the day that, Sensex plunges after company xys has announced it results.
To conclude, Plunge means a Fall