The Collaborative International Dictionary of English v.0.48:
Plunge \Plunge\, n.
1. The act of thrusting into or submerging; a dive, leap,
rush, or pitch into, or as into, water; as, to take the
water with a plunge.
2. Hence, a desperate hazard or act; a state of being
submerged or overwhelmed with difficulties. [R.]
She was brought to that plunge, to conceal her
husband's murder or accuse her son. --Sir P.
And with thou not reach out a friendly arm,
To raise me from amidst this plunge of sorrows?
3. The act of pitching or throwing one's self headlong or
violently forward, like an unruly horse.
4. Heavy and reckless betting in horse racing; hazardous
Plunge bath, an immersion by plunging; also, a large bath
in which the bather can wholly immerse himself.
Plunge battery, or plunging battery (Elec.), a voltaic
battery so arranged that the plates can be plunged into,
or withdrawn from, the exciting liquid at pleasure.