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.
If the property A' of person A damages the property B' of person B then this is an example of A initiating force against B because person A has taken ownership over property A by his will and therefore has responsibility over A'.
Examples of aggression
Spying is an example of aggression, because it requires the gaining of access to another persons property (force) without permission.