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.
I would say that sending potentially lethal toxins in the mail to politicians because of some bizarre grudge most likely qualifies as terrorism.
It goes without saying of course that not all terrorists are Arabs or Muslims. McVeigh and the UnaBomber were terrorists. And as much as it pains me to agree with HK on something, I have to agree that a letter describing oneself as "Fallen Angel" and complaining about trucking regulations definitely doesn't sound like an Al Queda type.
Of course HK doesn't think it's a Muslim even when the letter says "Death to America, Death to Israel, Allah is Great". He's really every bit as bad as the people he loves attacks for thinking that a terrorist could only be Muslim and nothing else.