The blood circle is a term used by the Boy Scouts to describe the area within the radius of the arm and blade length combined, when using a knife or ax. This area can be envisioned as a sphere with a person and a sharp instrument at its center. Presumably, anyone within this radius is at risk of being wounded by an accidental slip of the blade. Therefore, it is desirable to keep other people outside of the circle at all times.
Delimiting the blood circle
The circle is found by holding the blade of the cutting instrument (never the handle) in the hand used for cutting, and slowly swinging the arm in a 360° arc. The far end of the handle marks the limits of the circle. Any person or thing within this circle is considered to be at risk of injury. While it is never a good idea to enter another person's blood circle, it is customarily the responsibility of the knife-holder to clear the circle, or to move if the circle cannot be cleared.