Noun

1. An abrupt failure of function or complete physical exhaustion
2. A natural event caused by something suddenly falling down or caving in
3. The act of throwing yourself down
4. A sudden large decline of business or the prices of stocks (especially one that causes additional failures)

Verb

1. Break down, literally or metaphorically
2. Collapse due to fatigue, an illness, or a sudden attack
3. Fold or close up
4. Fall apart
5. Cause to burst
6. Suffer a nervous breakdown
7. Lose significance, effectiveness, or value