Noun

1. An eruption, sudden appearance

   Any epidemic outbreak causes understandable panic

2. A sudden increase.

   There has been an outbreak of head lice at the local school.

3. An outburst or sudden eruption, especially of violence and mischief.

   There has been an outbreak of broken windows in the street.

4. An uproar, riot

   Poorly trained officers can't control a prison outburst without excessive repression

5. A geological layer that breaks out

Verb

1. To burst out
2. To break forth