1. To break into pieces; SYN. fragment, fragmentize.
2. To cause to separate; SYN. disperse, scatter.
3. To come apart.
4. To separate (said of couples), usually definitively.
5. To laugh unrestrainedly; SYN. crack up.
6. To destroy the completeness of a set of related items; SYN. break up.

1. When you break up a fight, you stop the fight.
2. When a gathering of people separates, it breaks up. When the police tell people who are gathered together in a crowd to separate and leave the area, the police break the gathering up.
3. When something breaks into smaller pieces, it breaks up. When you break something into smaller pieces, you break it up.
4. When something breaks up the day or some other period of time, it interrupts that time and makes it less boring.