I would consider terrorism to be an ongoing campaign - more than one incident, whether that be violent acts or threatening acts. Terrorism to me has a political or social agenda with a group of supporters, campaigners and activists, although one person can act alone whilst being a representative of that group. Terrorists work for a cause and under a banner.
Terrorists claim group responsibility for their actions, so even if one or more of the group is arrested the terror continues - somebody else takes up the reins.
One person acting alone, even for a political or social cause, is not a terrorist. Once he is caught, he no longer poses a terrorist threat. It is ended.