Cleave:

Cleave is an attack effect that makes a hero’s physical attack damage spread in an area behind or around the target, allowing one hit to damage multiple nearby units. In Dota 2, cleave is used mainly on heroes and items that help clear creep waves, farm stacked camps, or hit clustered enemies in team fights.

Cleave usually applies only to attack damage, not most spell damage, and its area and effectiveness depend on the source that grants it.

Related: Farming, Area of Effect