There is clearly a wide choice of definitions for terrorism. Despite this, there are elements in common among the majority of useful definitions. Common threads of the various definitions identify terrorism as:
A terrorist act is a political act or is committed with the intention to cause a political effect. Clausewitz' statement that "war is a continuation of policy by other means" is taken as a truism by terrorists. They merely eliminate the intermediate step of armies and warfare, and apply violence directly to the political contest.
The intended results of terrorist acts cause a psychological effect ("terror"). They are aimed at a target audience other than the actual victims of the act. The intended target audience of the terrorist act may be the population as a whole, some specific portion of a society (an ethnic minority, for example), or decision-making elites in the society's political, social, or military populace.
Violence and destruction are used in the commission of the act to produce the desired effect. Even if casualties or destruction are not the result of a terrorist operation, the threat or potential of violence is what produces the intended effect. For example, a successful hostage taking operation may result in all hostages being freed unharmed after negotiations and bargaining. Regardless of the outcome, the terrorist bargaining chips were nothing less than the raw threat of applying violence to maim or kill some or all of the hostages. When the threat of violence is not credible, or the terrorists are unable to implement violence effectively, terrorism fails.
Terrorist groups demand change, revolution, or political movement. The radical worldview that justifies terrorism mandates drastic action to destroy or alter the status quo. Even if the goals of a movement are reactionary in nature, they require action to "turn back the clock" or restore some cherished value system that is extinct. Nobody commits violent attacks on strangers or innocents to keep things "just the way they are."
Terrorism is an activity planned and intended to achieve particular goals. It is a rationally employed, specifically selected tactic, and is not a random act. Since the victims of terrorist violence are often of little import, with one being as good for the terrorists' purposes as another, victim or target selection can appear random or unprovoked. But the target will contain symbolic value or be capable of eliciting emotional response according to the terrorists' goals. Remember that the actual target of terrorism is not the victim of the violence, but the psychological balance
Terrorism's effects are not necessarily aimed at the victims of terrorist violence. Victims are usually objects to be exploited by the terrorists for their effect on a third party. In order to produce this effect, information of the attack must reach the target audience. So any terrorist organization plans for exploitation of available media to get the message to the right audiences. Victims are simply the first medium that transmits the psychological impact to the larger target audience. The next step in transmission will depend on what media is available, but it will be planned, and it will frequently be the responsibility of a specific organization within the terrorist group to do nothing else but exploit and control the news cycle.
Some organizations can rely on friendly or sympathetic news outlets, but this is not necessary. News media can be manipulated by planning around the demands of the "news cycle", and the advantage that control of the initiative gives the terrorist. Pressures to report quickly, to "scoop" competitors, allow terrorists to present claims or make statements that might be refuted or critically commented on if time were available. Terrorists often provide names and details of individual victims to control the news media through its desire to humanize or personalize a story. For the victims of a terrorist attack, it is a certainty that the impact on the survivors (if there are any) is of minimal importance to the terrorists. What is important is the intended psychological impact that the news of their death or suffering will cause in a wider audience.
Operations in Permissive Societies
Terrorists conduct more operations in societies where individual rights and civil legal protections prevail. While terrorists may base themselves in repressive regimes that are sympathetic to them, they usually avoid repressive governments when conducting operations wherever possible. An exception to this case is a repressive regime that does not have the means to enforce security measures. Governments with effective security forces and few guaranteed civil liberties have typically suffered much less from terrorism than liberal states with excellent security forces. Al Qaeda has shown, however, that they will conduct operations anywhere.
Illegality of Methods
Terrorism is a criminal act. Whether the terrorist chooses to identify himself with military terminology (as discussed under insurgencies below), or with civilian imagery ("brotherhood", "committee", etc.), he is a criminal in both spheres. The violations of civil criminal laws are self-evident in activities such as murder, arson, and kidnapping regardless of the legitimacy of the government enforcing the laws. Victimizing the innocent is criminal injustice under a dictatorship or a democracy. If the terrorist claims that he is justified in using such violence as a military combatant, he is a de facto war criminal under international law and the military justice systems of most nations.
Preparation and Support
It's important to understand that actual terrorist operations are the result of extensive preparation and support operations. Media reporting and academic study have mainly focused on the terrorists' goals and actions, which is precisely what the terrorist intends. This neglects the vital but less exciting topic of preparation and support operations. Significant effort and coordination is required to finance group operations, procure or manufacture weapons, conduct target surveillance and analysis, and deliver trained terrorists to the operational area. While the time and effort expended by the terrorists may be a drop in the bucket compared to the amounts spent to defend against them, terrorist operations can still involve large amounts of money and groups of people. The need for dedicated support activities and resources on simple operations are significant, and get larger the greater the sophistication of the plan and the complexity of the target.