I don't regard a dictionary as holy writ, and I don't have any illusions that using one will settle these disputes, but I really do wish, when conducting arguments--particularly in writing via email--that contenders would resort more to the dictionary, at least as a starting point, for defining terms.
For example:
Terrorism: "The unlawful use or threatened use of force or violence by a person or an organized group against people or property with the intention of intimidating or coercing societies or governments, often for ideological or political reasons."
Terrorist: "One that engages in acts or an act of terrorism."
Not an answer, not unambiguous, not unchallengable, but a starting point.