plungephoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticphonetic verb
to (cause to) move or fall suddenly forward, down or into something
We ran down to the beach and plunged into the sea. [I]
He lost his footing while he was climbing the mountain, and plunged to his death. [I]
The car went out of control on a bend and plunged over the cliff. [I]

plungephoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticphonetic noun [C]
I really enjoyed my plunge (=jumping in and swimming) in the pool.
There has been a plunge in the (value of the) dollar (=it has suddenly fallen in value by a large amount) today.
We are expecting a plunge (=a large fall) in profits this year.

To take the plunge is determinedly to make a decision to do something, esp. after thinking about it for a long time.
I've decided to take the plunge and set up my own business.
They're finally taking the plunge and getting married.

plungingphoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticphonetic adjective
The Princess of Wales wore a black dress with a plunging neckline (=a dress shaped low at the neck so that part of her breasts could be seen).

This page is cited from Cambridge Dictionaries Online. For the original, please visit

http://dictionary.cambridge.org/define.asp?key=plunge*1+0