Terrorism is the use of violence or threats to coerce, especially for political reasons. It is also the use of such tactics in order to govern or to resist a government.

Crashing a plane into a government building because you hate government policies is definitely terrorism, I would say ...

Terrorism certainly doesn't have to be indiscriminate (bin Laden very specifically targeted the USS Cole, for example, and McVeigh picked his target pretty carefully). And terrorism doesn't depend on a belief system. Islamic terrorism is often belief-based, but not all terrorism is Islamic. All you have to do to be a terrorist is use violence or threats to attempt to affect policy, by stirring people against the people who make the policy, bringing attention to your grievences, and/or making people afraid to continue the policy for fear of further violence. Violent separatists are terrrorists. People who bomb nightclubs or subways or checkpoints to make people give in to a political demand are terrorists. And people who fly planes into government buildings because they hate the government are terrorists.

Yeah, this is just a lone, angry man who is mad at the world and the government, but technically I believe that what he did is still terrorism.