—v.t. 1. to cast or thrust forcibly or suddenly into something, as a liquid, a penetrable substance, a place, etc.; immerse; submerge: to plunge a dagger into one's heart. 2. to bring suddenly or forcibly into some condition, situation, etc.: to plunge a country into war; to pull a switch and plunge a house into darkness. 3. Hort.to place (a potted plant) up to its rim in soil or in certain other materials, as sand or moss.
4. Survey.to transit (the telescope of a transit or theodolite).
—v.i. 1. to cast oneself, or fall as if cast, into water, a hole, etc.
2. to rush or dash with headlong haste: to plunge through a crowd. 3. to bet or speculate recklessly: to plunge on the stock market. 4. to throw oneself impetuously or abruptly into some condition, situation, matter, etc.: to plunge into debt. 5. to descend abruptly or precipitously, as a cliff, road, etc.
6. to pitch violently forward, as a horse, ship, etc.
—n. 1. act of plunging.
2. a leap or dive, as into water.
3. a headlong or impetuous rush or dash: a plunge into danger. 4. a sudden, violent pitching movement.
5. a place for plunging or diving, as a swimming pool.
6. Geol.pitch (def. 48).
7. take the plunge, to enter with sudden decision upon an unfamiliar course of action, as after hesitation or deliberation: She took the plunge and invested her entire savings in the plan.