plunge

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To dive, jump, or throw oneself.
• intransitive verb To fall rapidly.
• intransitive verb To devote oneself to or undertake an activity earnestly or wholeheartedly.
• intransitive verb To enter or move headlong through something.
• intransitive verb To slope steeply downward.
• intransitive verb To move forward and downward violently.
• intransitive verb To become suddenly lower; decrease dramatically.
• intransitive verb To thrust or throw forcefully into a substance or place.
• intransitive verb To cast suddenly, violently, or deeply into a given state or situation.
• intransitive verb To use a plunger to try to unblock (a drain, for example).
• noun The act or an instance of plunging.
• noun A swim; a dip.
• noun A sudden or dramatic decline.
• idiom (take the plunge) To begin an unfamiliar venture, especially after hesitating.

from The Century Dictionary.

• noun A sudden dive, leap, or dip into something: as, a plunge in the sea.
• noun An immersion in difficulty, embarrassment, or distress; the condition of being surrounded or overwhelmed; a strait; difficulty.
• noun A sudden and violent pitching forward of the body, and pitching up of the hind legs, as by an unruly horse
• To cast or thrust suddenly into water or some other fluid, or into some penetrable substance; immerse; thrust: as, to plunge one's hand into the water; to plunge a dagger into one's breast.
• Figuratively, to cast or throw into some thing, state, condition, or action: as, plunged in grief; to plunge a nation into war.
• To entangle or embarrass: used chiefly in the past participle.
• To dive, leap, or rush (into water or some fluid).
• To fall or rush headlong into some thing, action, state, or condition: as, to plunge into debt or into a controversy.
• To throw the body forward and the hind legs up, as an unruly horse.
• To descend precipitously or vertically, as a cliff.
• To bet recklessly; gamble for large stakes; speculate.
• In horticulture, to sink (a pot or box containing a plant) in the ground to the rim or edge. Pots of greenhouse plants are often plunged in the open in warm weather, both for the good of the plants and for their effect in ornamentation.
• To turn over (the telescope of a surveyor's transit or theodolite) in a vertical plane, making the object-glass pass underneath. In transiting the telescope it may pass either above or below.
• In geology, to dip under the surface: used in reference to such structural features as folds where, unless the axis is perfectly horizontal, one end pitches below the horizon or general surface.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To thrust or cast one's self into water or other fluid; to submerge one's self; to dive, or to rush in. Also used figuratively.
• intransitive verb To pitch or throw one's self headlong or violently forward, as a horse does.
• intransitive verb Cant To bet heavily and with seeming recklessness on a race, or other contest; in an extended sense, to risk large sums in hazardous speculations.
• intransitive verb (Gun.) firing directed upon an enemy from an elevated position.
• transitive verb To thrust into water, or into any substance that is penetrable; to immerse; to cause to penetrate or enter quickly and forcibly; to thrust. Also used figuratively.
• transitive verb To baptize by immersion.
• transitive verb obsolete To entangle; to embarrass; to overcome.
• noun The act of thrusting into or submerging; a dive, leap, rush, or pitch into, or as into, water.
• noun rare Hence, a desperate hazard or act; a state of being submerged or overwhelmed with difficulties.
• noun The act of pitching or throwing one's self headlong or violently forward, like an unruly horse.
• noun Cant Heavy and reckless betting in horse racing; hazardous speculation.
• noun an immersion by plunging; also, a large bath in which the bather can wholly immerse himself.
• noun (Elec.) a voltaic battery so arranged that the plates can be plunged into, or withdrawn from, the exciting liquid at pleasure.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the act of plunging or submerging
• noun a dive, leap, rush, or pitch into (into water)
• noun figuratively the act of pitching or throwing one's self headlong or violently forward, like an unruly horse
• noun slang heavy and reckless betting in horse racing; hazardous speculation
• noun obsolete an immersion in difficulty, embarrassment, or distress; the condition of being surrounded or overwhelmed; a strait; difficulty
• verb transitive to thrust into water, or into any substance that is penetrable; to immerse;
• verb figuratively, transitive to cast or throw into some thing, state, condition or action