Every age has its enemies. In the mid-20th century, Fascists were the evildoers. After WWII, Communists became civilization's nemesis. Now terrorists have become the designated masters of malevolence. The word "terrorism" appears in law books and legislation around the world. Various civil sanctions apply to "terrorist organizations," and it can be a crime to assist one.
But it is not always easy to determine who "they"--the terrorists--are. Whether organizations are terrorist or not is largely a determination motivated by politics. The UN repeatedly passes resolutions against terrorism, but cannot agree on how to define the term.
Official definitions of terrorism are unpersuasive. The US Congress, for example, defines terrorism as including a motive to coerce or intimidate a population or influence a government. But this formula does not clearly cover even the terrorist attacks of September 2001. If the motive of the airline hijackers was simply to kill infidels, their attack would fall outside the Congressional definition.
It is a mistake to try to define terrorism in the same way that we define theft or murder. There are too many contested issues. A better approach is to identify the issues that arise in thinking about terrorism and clarify why people experience terror from certain acts of violence. Then we can define terrorism with reference to all these variables, but without making any one of them decisive.