Aggression is defined by classical liberals 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, while this person did not have permission. The relation between force and will is called causation.
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.
Note that communication requires the initiation of force to gain access to another persons sensory organs. If politeness is kept, permission for communication is usually given after the fact.