The best definition we have for terrorism is the threat or use of illegal violence to influence the people beyond the immediate area of the attack to achieve a political, religious, ideological or monetary goal.
It’s not necessarily making a moral judgement about terrorism, just how it works. Al Qaeda is a terrorist organization because they used violence and threatened more with the goal of influencing US foreign policy on the basis of Fundamentalist Islamic theology. The IRA were also terrorists for their use and threats of violence with the political goal of unifying Ireland as one nation and taking it back from the British monarchy. Even the Founding Father’s of the US could be considered terrorists by this definition. They destroyed property and killed British citizens in our bid for independence affecting the decisions of the monarchy and parliament an entire ocean away.