I would suggest that terrorisim is well outside the bounds of warfare... as defined by Sun Tzu, US Grant, RE Lee, George Patton, or any RESPECTED warfighter, past or present.
Terrorism is blood lust, vengance, hatred personified and transformed into an act. Those who practice it try to legimitize it as a tactic of war.
Lets not redefine terrorism.
Terrorism: the calculated use of violence (or the threat of violence) against civilians in order to attain goals that are political or religious or ideological in nature; this is done through intimidation or coercion or instilling fear.
Any attempt to achieve goals by terrorizing civilians is terrorism, irrelevant of what those goals are.
...as for no respected warfighter using it:
Sherman (and his march ordered by above mentioned Grant).
...and I'm sure Lee knew nothing about the confederate attempts to firebomb hotels in NYC during the election of 1864.
Never mind things like the Dresden and Tokyo firebombings.