Aggression

Aggression is defined as the initiation of physical force against other persons or their property, the threat of such, or fraud upon other persons or their property.

Synonymous to the word physical force, simply the word force or the word violence is often used. Strictly speaking force is only violence when it is used against another persons property instead of ones own property.

The word initiation means that the force can be causally traced back to the will of a person, where this person did not have permission. So a person did not initiate the use of force when he was just a domino in a causal chain that he did not initiate by his will.