I happen to think terrorism is a adequately well-defined by sort of inspecting the word itself: terrorism is any systematic (thus the "ism") act or acts that are meant to inspire a particular emotion - terror. The most bizarre facet of terrorism is that it's a crime of inflicting an emotion, an affective crime. For example, it's rare to have inflicting emotions be criminalized - the police tend not to drag people away to jail for inflicting joy upon someone, or remorse, contempt, lust...
Since terrorism is primarily the inflicting of an emotion, terrorism is a subjective act, differing from more normal "crimes" like murder or thievery because very rarely are there traditionally physical facts of the matter - i.e. dead bodies, or CCTV footage with items not being paid for. Since it's subjective, terrorism is only terrorism in the eyes (at least if it's successful) of those who actually are inflicted by terrorism. So Al-Qaeda was pretty effective in causing quite a few of government officials in the USA to feel terror on September 11th, but lots of other people were indifferent or fascinated (for example, blacks in the ghettos around DC I was with at the time..) but certainly not feeling terror, and for people who agreed with Al-Qaeda, I'm sure there was no small feeling of joy. I think a lot of this slipperiness of the word "terrorism" comes from the fact it's primarily an affective, subjective, emotional "crime".
So terrorism per se has nothing inherently to do with violence or evil - unless you happen to be one of the people that feels the emotion of terror, and violence does tend to inflict terror. But then some people find clowns scary, and for many types of terror there's no physical violence involved. Any attempt to universalize the emotion of terror is simply an attempt to coerce people into feeling the same emotion (perhaps as yourself), and is thus often terrorism itself.