It isn't terrorism if you are attacking an armed military target
Especially under conditions of war. The line is not a solid one. In a full-scale conventional war between nations, power plants and industrial facilities are legitimate targets since they support the war effort, yet they are not military. It needs to be thought about on a case-by-case basis. Some are very hard to classify.
The attack on the WTC was terrorism.
Guerilla attacks on coalition military convoys are not.
The attack on the Red Cross was terrorism.
Suicide bombings in cafes in Israel are terrorism.
Attacking known Hamas personnel is not.