Deadly force is force that has the capacity to kill a human being. Examples of deadly force include shooting a gun at a person, or swinging a baseball bat at a person's head. Deadly force is important to many laws. For example, there are certain circumstances in which both police officers and regular citizens are authorized to use deadly force. Self defense in a life threatening situation is one such circumstance.
Deadly force is basically just when a particular action has the potential to cause death. Deadly force is important to law because it can help to determine the severity of a crime. For example, assault with a deadly weapon (a weapon that can cause deadly force) is more serious than assault without one. Deadly force is also relevant to law enforcement. The reason is because there are some circumstances where police officers may be required to use deadly force. For example, deadly force may be required if officers are chasing bank robbers who are shooting at them.